Wirz

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Journal Publications

2024

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McKenna JD Breddan, Richard E Wirz “Machine learning electrospray plume dynamics,” Engineering Applications of Artificial Intelligence 133 (2024); https://doi.org/10.1016/j.engappai.2024.108095

Machine learning models are applied to simulated electrospray particle data to investigate plume dynamics from emission to final particle properties. A limited set of final particle properties are successfully regressed exclusively from emission property inputs. Random Forest model feature rankings for final plume angle reveal that particle charge has dominant influence when emission velocity is strictly axial, while lateral emission velocity has dominant influence when particles are emitted with an off-axis velocity component. In addition to providing correlations between initial and final particle properties, the machine learning models also identify correlations between different final particle properties. These correlations reveal opportunities for experimental approaches and diagnostic design by determining experimental measurements that offer insight into desired final particle properties.


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Jorge Fernandez-Coppel, Richard Wirz, Jaime Marian, “Fully discrete model of kinetic ion-induced electron emission from metal surfaces,” Journal of Applied Physics 135(8) (2024); https://doi.org/10.1063/5.0188000

Ion-induced electron emission (IIEE) is an important process whereby ions impinging on a material surface lead to net emission of electrons into the vacuum. While relevant for multiple applications, IIEE is a critical process of electric thruster (ET) operation and testing for space propulsion, and, as such, it must be carefully quantified for safe and reliable ET performance. IIEE is a complex physical phenomenon, which involves a number of ion-material and ion-electron processes, and is a complex function of ion mass, energy, and angle, as well as host material properties, such as mass and electronic structure. In this paper, we develop a discrete model of kinetic IIEE to gain a more accurate picture of the electric thruster chamber and facility material degradation processes. The model is based on three main developments: (i) the use of modern electronic and nuclear stopping databases, (ii) the use of the stopping and range of ions in matter to track all ion and recoil trajectories inside the target material, and (iii) the use of a scattering Monte Carlo approach to track the trajectories of all mobilized electrons from the point of first energy transfer until full thermalization or escape. This represents a substantial advantage in terms of physical accuracy over existing semi-analytical models commonly used to calculate kinetic IIEE. We apply the model to Ar, Kr, and Xe irradiation of W and Fe surfaces and calculate excitation spectra as a function of ion depth, energy, and angle of incidence. We also obtain minimum threshold ion energies for net nonzero yield for each ion species in both Fe and W and calculate full IIEE yields as a function of ion energy and incidence angle. Our results can be used to assess the effect of kinetic electron emission in models of full ET facility testing and operation.


2023

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Shehan M Parmar, Daniel D Depew, Richard E Wirz, Ghanshyam L Vaghjiani, “Structural Properties of HEHN-and HAN-Based Ionic Liquid Mixtures: A Polarizable Molecular Dynamics Study,” The Journal of Physical Chemistry B 170(40) (2023); https://doi.org/10.1021/acs.jpcb.3c02649

Molecular dynamics simulations of binary mixtures comprising 2-hydroxyethylhydrazinium nitrate (HEHN) and hydroxylammonium nitrate (HAN) were conducted using the polarizable APPLE&P force field to investigate fundamental properties of multimode propulsion (MMP) propellants. Calculated densities as a function of temperature were in good agreement with experiments and similar simulations. The structural properties of neat HEHN and HAN–HEHN provided insights into their inherent, protic nature. Radial distribution functions (RDFs) identified key hydrogen bonding sites located at N–H···O and O–H···O within a first solvation shell of approximately 2 Å. Angular distribution functions further affirmed the relatively strong nature of the hydrogen bonds with nearly linear directionality. The increased hydroxylammonium cation (HA+) mole fraction shows the influence of competitively strong hydrogen bonds on the overall hydrogen bond network. Dominant spatial motifs via three-dimensional distribution functions along with nearly nanosecond-long hydrogen bond lifetimes highlight the local bonding environment that may precede proton transfer reactions.


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Yuchen Qian, Walter Gekelman, Patrick Pribyl, Tom Sketchley, Shreekrishna Tripathi, Zoltan Lucky, Marvin Drandell, Stephen Vincena, Thomas Look, Phil Travis, Troy Carter, Gary Wan, Mattia Cattelan, Graeme Sabiston, Angelica Ottaviano, Richard Wirz, “Design of the Lanthanum hexaboride based plasma source for the large plasma device at UCLA,” Review of Scientific Instruments 94(8) (2023); PDF, https://doi.org/10.1063/5.0152216

The Large Plasma Device (LAPD) at UCLA (University of California, Los Angeles) produces an 18 m long, magnetized, quiescent, and uniform plasma at a high repetition rate to enable studies of fundamental plasma physics. Here, we report on a major upgrade to the LAPD plasma source that allows for more robust operation and significant expansion of achievable plasma parameters. The original plasma source made use of a heated barium oxide (BaO) coated nickel sheet as an electron emitter. This source had a number of drawbacks, including a limited range of plasma density (≲ 4.0× 1012 cm− 3), a limited discharge duration (∼ 10 ms), and susceptibility to poisoning following oxygen exposure. The new plasma source utilizes a 38 cm diameter lanthanum hexaboride (LaB6) cathode, which has a significantly higher emissivity, allowing for a much larger discharge power density, and is robust to exposure to air …


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Angelopoulos, V., Zhang, XJ., Artemyev, A.V. et al. “Energetic Electron Precipitation Driven by Electromagnetic Ion Cyclotron Waves from ELFIN’s Low Altitude Perspective,” Space Sci Rev 219, 37 (2023); https://doi.org/10.1007/s11214-023-00984-w

We review comprehensive observations of electromagnetic ion cyclotron (EMIC) wave-driven energetic electron precipitation using data collected by the energetic electron detector on the Electron Losses and Fields InvestigatioN (ELFIN) mission, two polar-orbiting low-altitude spinning CubeSats, measuring 50-5000 keV electrons with good pitch-angle and energy resolution. EMIC wave-driven precipitation exhibits a distinct signature in energy-spectrograms of the precipitating-to-trapped flux ratio: peaks at >0.5 MeV which are abrupt (bursty) (lasting ∼17 s, or ) with significant substructure (occasionally down to sub-second timescale). We attribute the bursty nature of the precipitation to the spatial extent and structuredness of the wave field at the equator. Multiple ELFIN passes over the same MLT sector allow us to study the spatial and temporal evolution of the EMIC wave - electron interaction region. Case studies employing conjugate ground-based or equatorial observations of the EMIC waves reveal that the energy of moderate and strong precipitation at ELFIN approximately agrees with theoretical expectations for cyclotron resonant interactions in a cold plasma. Using multiple years of ELFIN data uniformly distributed in local time, we assemble a statistical database of ∼50 events of strong EMIC wave-driven precipitation. Most reside at at dusk, while a smaller subset exists at at post-midnight. The energies of the peak-precipitation ratio and of the half-peak precipitation ratio (our proxy for the minimum resonance energy) exhibit an -shell dependence in good agreement with theoretical estimates based on prior statistical observations of EMIC wave power spectra. The precipitation ratio’s spectral shape for the most intense events has an exponential falloff away from the peak (i.e., on either side of MeV). It too agrees well with quasi-linear diffusion theory based on prior statistics of wave spectra. It should be noted though that this diffusive treatment likely includes effects from nonlinear resonant interactions (especially at high energies) and nonresonant effects from sharp wave packet edges (at low energies). Sub-MeV electron precipitation observed concurrently with strong EMIC wave-driven >1 MeV precipitation has a spectral shape that is consistent with efficient pitch-angle scattering down to ∼ 200-300 keV by much less intense higher frequency EMIC waves at dusk (where such waves are most frequent). At ∼100 keV, whistler-mode chorus may be implicated in concurrent precipitation. These results confirm the critical role of EMIC waves in driving relativistic electron losses. Nonlinear effects may abound and require further investigation.


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Zhitong Chen, Richard Obenchain, Richard E Wirz, “Cold plasma treatment for biomedical applications: using aluminum foam to reduce risk while increasing efficacy,” arXiv preprint arXiv:2305.18349 (2023); pdf

Plasma medicine is an emerging and innovative interdisciplinary research field combining biology, chemistry, physics, engineering, and medicine. However, the safe clinical application of cold atmospheric plasma (CAP) technology is still a challenge. Here, we examine the use of aluminum (Al) foam with three pores-per-inch (PPI) ratings in clinical plasma applications. Al foams can filter sparks to avoid damage from high voltage discharge during surgery and efficiently deliver reactive species generated in CAP to the target. The sparks appear and plasma intensity increases at the foam/discharge interface, which just slightly increases the interface temperature without changing the interface microstructure during a 30-minute treatment. After CAP penetrated the Al foams, N2, N2+, *OH, O, and He emission peaks were characterized, and the highest values appeared using Al foams with 10 PPI. CAP with and without Al foam intermediating was used to treat deionized water, and the results indicate CAP in combination with 10 PPI Al foam led to much higher ROS concentration than CAP alone. For melanoma cell experiments, CAP with and without Al foam had a similar effect on cell viability after 30-second treatment, while CAP with the 10-PPI Al foam had much higher killing efficiency than CAP alone after 60-second treatment. In summary, 10-PPI Al foam can not only prevent damage to tissues resulting from high discharge voltage during clinical surgery but also increase the delivery efficiency of reactive species generated in plasma for biomedical applications.


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Ottaviano, A., & Wirz, R. E., “Secondary electron emission of reticulated foam materials,” Journal of Applied Physics, 133(10) (2023); https://doi.org/10.1063/5.0133253

Complex material surfaces can reduce secondary electron emission (SEE) and sputtering via geometric trapping. In this work, the SEE yields for a range of open-cell reticulated carbon foam geometries are characterized using scanning electron microscopy. The total reduction in the SEE yield from carbon foams with a 3% volume fill density and 10–100 pores per inch (PPI) is shown to be between 23.5% and 35.0%. Contributions of a foam backplate are assessed by experimentally and analytically defining the critical parameter, transparency. The transparency of a foam is quantified and is shown to affect the primary electron angular dependence on the SEE yield. For the same thickness of 6 mm, it is found that higher PPI decreases foam transparency from 32% to 0% and reduces the SEE yield. The SEE yield from carbon foams is also shown to have weaker dependence on the morphology of the surface compared with fuzzes and velvets and less variation across individual sample surfaces due to the rigidity of their ligament structures and isotropic geometries.


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NM Uchizono, RE Wirz, AL Collins, C Marrese-Reading, SM Arestie, JK Ziemer, “A diagnostic for quantifying secondary species emission from electrospray devices,” Review of Scientific Instruments, 92(2) (2023); pdf

Measuring the polydisperse beam of charged species emitted by an electrospray device requires accurate measurements of current. Secondary species emission (SSE) caused by high velocity nanodroplet or molecular ion impacts on surfaces contributes to substantial uncertainty in current measurements. SSE consists of both positive and negative species, so mitigating measurement uncertainty requires different considerations than plasma diagnostic techniques. The probe and analysis methods described herein distinguish between current contributions from positive SSE, negative SSE, and primary species. Separating each contribution provides positive and negative SSE yield measurements, and corrected current measurements that reflect the true primary current. Sources of measurement uncertainty in probe design are discussed, along with appropriate mitigation methods. The probe and analysis technique are demonstrated on an ionic liquid electrospray operating in droplet emission mode to obtain an angular distribution of positive and negative SSE yields for an ionic liquid electrospray.


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Mary F Konopliv, Vernon H Chaplin, Lee K Johnson, Richard E Wirz, “Accuracy of using metastable state measurements in laser-induced fluorescence diagnostics of xenon ion velocity in Hall thrusters,” Plasma Sources Science and Technology, Volume 32 (2023); pdf

Laser-induced fluorescence measurements of singly-charged xenon ion velocities in Hall thrusters typically target metastable states due to lack of available laser technology for exciting the ground state. The measured velocity distribution of these metastable ions are assumed to reflect the ground state ion behavior. However, this assumption has not been experimentally verified. To investigate the accuracy of this assumption, a recently developed xenon ion (Xe II) collisional-radiative model is combined with a 1D fluid model for ions, using plasma parameters from higher fidelity simulations of each thruster, to calculate the metastable and ground state ion velocities as a function of position along the channel centerline. For the HERMeS and SPT-100 thruster channel centerlines, differences up to 0.5 km s−1 were observed between the metastable and ground state ion velocities. For the HERMeS thruster, the difference between the metastable and ground state velocities is less than 150 m s−1 within one channel length of the channel exit, but increases thereafter due to charge exchange (CEX) that reduces the mean velocity of the ground state ions. While both the ground state ions and metastable state ions experience the same acceleration by the electric field, these small velocity differences arise because ionization and CEX directly into these states from the slower neutral ground state can reduce their mean velocities by different amounts. Therefore, the velocity discrepancy may be larger for thrusters with lower propellant utilization efficiency and higher neutral density. For example, differences up to 1.7 km s−1 were calculated on the HET-P70 thruster channel centerline. Note that although the creation of slow ions can influence the mean velocity, the most probable velocity should be unaffected by these processes. Keywords: metastable, diagnostics, electric prop


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McKenna J.D. Breddan, Richard E. Wirz, “Electrospray plume evolution: Influence of drag,” Journal of Aerosol Science, Volume 167 (2023); https://doi.org/10.1016/j.jaerosci.2022.106079

The University of California, Los Angeles (UCLA) Plasma, Energy, & Space Propulsion Laboratory (PESPL) presents the Discrete Electrospray Lagrangian Interaction (DELI) Model for simulating the evolution of electrospray plumes. This publication describes the DELI Model, verification of its Coulomb collision module, model validation with atmospheric plume data, and novel comparisons of simulated plumes evolved with different fractions of the drag force. DELI Model results reproduce experimentally-observed droplet clustering events that yield plume expansion as a result of Coulomb repulsion. Furthermore, the presented comparison of identical emitted species evolved to steady state with different fractions of applied drag force demonstrates decreased mean droplet velocity and increased plume expansion with increased drag force. Publication results serve as a useful tool in examining electrospray plume evolution in atmospheric and vacuum regimes. Keywords: Electrospray; Plume evolution; Coulomb expansion; Drag deceleration


2022

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Collins, A.L., Wright, P.L., Uchizono, N.M. et al., “High angle mass flux of an electrospray plume,” J Electr Propuls 1, 32 (2022); https://doi.org/10.1007/s44205-022-00031-w

High-resolution mass flux measurements of an electrospray plume are reported at high (>30°) angles, which are relevant to direct impingement of downstream electrodes. Interrogation of the plume edge greatly reduces uncertainty in electrospray device lifetime estimation related to mass flux to electrode surfaces. An angularly-actuated Thermoelectric Quartz Crystal Microbalance (TQCM) provides resolution down to 2 pg cm² s⁻¹, allowing the highest resolution mass flux measurements of an electrospray plume to be reported herein. In-situ microscopy of the electrospray meniscus revealed changes to the electrode lines-of-sight of approximately 2°–3° due to the increasing meniscus tip height with beam current. Using the TQCM measurements and previous QCM results, a data-driven model is proposed for estimating electrode impingement as a function of beam current and aperture line-of-sight, which quantitatively captures the rapid increase in mass flux at higher beam currents or at low angles. The results show it is possible to guarantee negligible electrode impingement within a specified range of throttle levels.


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Huh, H., & Wirz, R. E., “Simulation of electrospray emission processes for low to moderate conductivity liquids,” Physics of Fluids, 34(11) (2022); https://doi.org/10.1063/5.0120737

An electrohydrodynamic numerical model is used to explore the electrospray emission behavior of both moderate and high electrical conductivity liquids under electrospray conditions. The Volume-of-Fluid method, incorporating a leaky-dielectric model with a charge relaxation consideration, is used to conserve charge to accurately model cone-jet formation and droplet breakup. The model is validated against experiments and agrees well with both droplet diameters and charge-to-mass ratio of emitted progeny droplets. The model examines operating conditions such as flow rate and voltage, with fluid properties also considered, such as surface tension, electrical conductivity, and viscosity for both moderate and high conductivity. For high conductivity and surface tension, the results show that high charge concentration along with the meniscus and convex cone shape results in a higher charge-to-mass ratio of the emitted droplets while lower conductivity and surface tension tend towards concave cone shapes and lower charge-to-mass droplets. Recirculation flows inside the bulk liquid are investigated across a range of non-dimensional flow rates, δ, and electric Reynolds numbers, ReE. For high conductivity liquid emission at the minimum stable flow rate, additional recirculation cells develop near the cone tip suggesting the onset of the axisymmetric instability.


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Wright, P., Wirz, R., “Transient Flow in Porous Electrosprays,” (2022); https://doi.org/10.21203/rs.3.rs-1865748/v1

Porous ionic electrospray emitters have received significant interest for space propulsion due to their performance and operational simplicity. We have developed a diffusion equation for describing the transient flow response in a porous electrospray emitter, which allows for the prediction of the settling time for flow in the porous emitter. This equation accounts for both the change in liquid storage at exposed pores on the emitter with pressure, and viscous diffusion through Darcy’s law. Transient flow solutions are provided for the most common emitter topologies: pillar, cone, and wedge. Transient flow solutions describe the settling time and magnitude of current overshoot from porous electrosprays. Comparing diffusion of pressure to the onset delay model for electrospray emission shows that diffusion is most relevant at higher voltages and when a porous reservoir is used. Accounting for multiple emission sites on the wedge geometry shows that emission sites settle in proportion to emission site spacing to the power -1.74. Applying the diffusion equation to published results shows good agreement between analytical predictions and experimental data.


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Uchizono, N., Wright, P., Collins, A., et al., “Emission spectra of glows produced by ionic liquid ion sources,” Appl. Phys. Lett. 121, 154101 (2022); https://doi.org/10.1063/5.0096595

Electrospray devices, such as ionic liquid ion sources, often exhibit glows during operation in high vacuum facilities. The lack of electron excitation mechanisms during electrospray operation prompts the question: “What causes glow in an electrospray device?” Our optical emission spectroscopy results show that electrospray glow exhibits a broad spectral response between 350 and 800 nm with emission lines corresponding to atomic metal constituents of impinged surfaces, neutral and ionized atomic constituents of the ionic liquid propellant, and molecular line shapes that may also be dissociation products of the ionic liquid. We have previously defined secondary species emission to describe the many complex interfacial phenomena that occur when electrosprayed species impact surfaces downstream of the emitter. Our analysis of the optical emission spectra shows that these glows are only possible in the presence of secondary species emission. Therefore, the answer to the proposed question: high-velocity impacts that generate secondary species are the root cause of glow for electrosprays in high vacuum facilities.


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Enomoto, T., Parmar, S.M., Yamada, R. et al., “Molecular Dynamics Simulations of Ion Extraction from Nanodroplets for Ionic Liquid Electrospray Thrusters,” J Electr Propuls 1, 13 (2022); https://doi.org/10.1007/s44205-022-00010-1

Molecular dynamics (MD) simulations were performed for ion extraction from electrospray thrusters to investigate relevant extraction processes numerically. To approximate the electrospray jet tip, a simulation domain consisting of 4-5 nm-sized ionic liquid droplets was used. The extracted ion angles and kinetic energies from EMI–BF4 (1-ethyl-3-methylimidazolium tetrafluoroborate) and EMI–Im (1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide) droplets were quantified by applying uniform electric fields of 1.3–1.7 V nm−1. The MD simulations are in great agreement with simulations presented in the literature and consistently show a greater preference for monomer emission than reported experimentally. At field strengths above 1.5 V nm−1, apparent droplet fracturing and breakup lead to an increase in ion angular velocity distributions. Greater mobility of EMI–BF4 ions than EMI–Im was also observed, indicative of the crucial role of cation-anion hydrogen bond strengths in ion extraction and beam composition between different propellants.


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Crandall, P., Wirz, R.E., “Air-breathing electric propulsion: mission characterization and design analysis.,” J Electr Propuls 1, 12 (2022); https://doi.org/10.1007/s44205-022-00009-8

Air breathing electric propulsion (atmosphere-breathing electric propulsion) (ABEP) has attracted significant interest as an enabling technology for long duration space missions in very low Earth orbit (VLEO) altitudes below about 300 km. The ABEP spacecraft and mission analysis model developed allows parametric characterization of key spacecraft geometry and thruster performance parameters such as spacecraft length-to-diameter, the ratio of solar array span to spacecraft diameter, thrust-to-power, effective exhaust velocity, and inlet efficiency. For the missions analyzed ABEP generally outperforms conventional electric propulsion (EP) below 250 km altitude. Using a 6U spacecraft architecture the model shows that below 220 km ABEP is the only viable propulsion option for desirable mission lifetimes. Parametric evaluations of key spacecraft and ABEP characteristics show that the most significant technological improvements to ABEP spacecraft performance and range of applicability for VLEO missions will come from advancements in inlet efficiency, low drag materials, solar array efficiency, and thrust-to-power.


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Chen, Z., Bai, F., Jonas, S. J., & Wirz, R. E.,“Cold atmospheric plasma for addressing the COVID‐19 pandemic,” Plasma Processes and Polymers, 19(9) (2022); https://doi.org/10.1002/ppap.202200012

The coronavirus disease 2019 (COVID‐19) pandemic has greatly stressed the global community, exposing vulnerabilities in the supply chains for disinfection materials, personal protective equipment, and medical resources worldwide. Disinfection methods based on cold atmospheric plasma (CAP) technologies offer an intriguing solution to many of these challenges because they are easily deployable and do not require resource‐constrained consumables or reagents needed for conventional decontamination practices. CAP technologies have shown great promise for a wide range of medical applications from wound healing and cancer treatment to sterilization methods to mitigate airborne and fomite transfer of viruses. This review engages the broader community of scientists and engineers that wish to help the medical community with the ongoing COVID‐19 pandemic by establishing methods to utilize broadly applicable CAP technologies.


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Uchizono, N. M., Marrese-Reading, C., Arestie, S. M., Collins, A. L., Ziemer, J. K., & Wirz, R. E., “Positive and negative secondary species emission behavior for an ionic liquid electrospray,” Applied Physics Letters, 121(7) (2022); https://doi.org/10.1063/5.0102592

Ionic liquid electrosprays can emit a polydisperse population of charged droplets, clusters, and molecular ions at high velocity. Secondary species emission (SSE) is a term that encompasses the many concurrent impact and emission phenomena that occur when electrosprayed primary species strike a surface, resulting in a diverse population of secondary electrons, ions, clusters, and droplets. This letter examines the spatial dependency of SSE behavior across an [EMI]Im electrospray beam using microscopy of the target surface, and experimental quantification of SSE yields as a function of plume angle. Microscopy of the beam target confirms our prediction of shock-induced desorption when operating at elevated beam voltages. SSE yield measurements show that, upon impact with a surface, incident primary species that consist of entirely positive charge will produce both positive and negative SSE. Furthermore, results show that the SSE yields for an ionic liquid electrospray have strong spatial and energy dependencies. These findings have significant implications for understanding and predicting ionic liquid electrospray thruster lifetime and performance, and focused ion beam applications.


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Chen, Z., Chen, G., Obenchain, R., Zhang, R., Bai, F., Fang, T., Wang, H., Lu, Y., Wirz, R. E., & Gu, Z., “Cold atmospheric plasma delivery for biomedical applications,” Materials Today, 54, 153–188 (2022); https://doi.org/10.1016/j.mattod.2022.03.001

As the fourth state of matter, plasma’s unique properties and interactions with other states of matter offer many promising opportunities for investigation and discovery. In particular, cold atmospheric plasma (CAP), operating at atmospheric pressure and room temperature, has remarkable potential for biomedical applications through various delivery methods. These biomedical applications include sterilization, wound healing, blood coagulation, oral/dental diseases treatment, cancer therapy, and immunotherapy. Effective delivery of plasma constituents is critical to its efficacy for these applications. Therefore, this review presents the key research activities related to CAP delivery (including direct CAP delivery, delivery of plasma-activated media, biomedical device-assisted plasma delivery, and CAP delivery with other therapeutics) and needs for future research. This review will be of great interest for understanding the current state-of-the-art of biomedical applications of plasma medicine while also giving researchers from a broad range of communities insight into research efforts that would benefit from their contributions. Such communities include biomedicine, physics, biochemistry, material science, nanotechnology, and medical device manufacturing.


2021

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Uchizono N.M., Collins A.L, Marrese-Reading C., Arestie S.M., Ziemer J.K., Wirz R.E., “The Role of Secondary Species Emission in Vacuum Facility Effects for Electrospray Thrusters,” J. Applied Physics 130, 143301 (2021); https://doi.org/10.1063/5.0063476

Theoretical, analytical, and experimental investigations of electrospray operation in vacuum facilities show that secondary species emission (SSE) plays a significant role in the behavior of electrospray thrusters during ground testing. A review of SSE mechanisms, along with an analysis of onset thresholds for electrospray thruster conditions, indicates that secondary species (e.g., electrons, anions, cations, etc.) must be carefully considered for accurate measurements and determination of performance and life. Presented models and experiments show that SSE-induced thruster-to-facility coupling can lead to considerable measurement uncertainty but can be effectively mitigated with an appropriate beam target design. The Electrospray SSE Control-volume Analysis for Resolving Ground Operation of Thrusters model is applied to experimental data to analyze SSE behavior. A heat and mass flux analysis of the Air Force Electrospray Thruster Series 2 (AFET-2) shows that SSE-induced Ohmic dissipation can cause performance limitations in ionic liquid ion source thrusters. The presented analytical models show that backstreaming current density contributing to less than 0.1% of measured emitter current density can cause substantial variation in propellant properties. Additionally, backstreaming current density contributing to less than 3% of emitted current can cause the 0.86ugs^-1 neutral loss rate estimated during AFET-2 testing. Arguments are presented to support the notion that glow discharges observed in electrospray thrusters during vacuum operation are a consequence of secondary species backstreaming to the emission site, rather than a process intrinsically caused by ion evaporation. Recommendations for general best practices to minimize the effects of SSE on electrospray thruster operation are provided.


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Chen G., Chen Z., Wang Z., Obenchain R., Wen D., Li H., Wirz R.E., Gu Z., “Lightning-mimicking cold atmospheric plasma for post-surgical cancer treatment,” Science Advances, Vol 7, Issue 36 (2021); https://www.science.org/doi/10.1126/sciadv.abg5686

Surgery represents the major option for treating most solid tumors. Despite continuous improvements in surgical techniques, cancer recurrence after surgical resection remains the most common cause of treatment failure. Here, we report cold atmospheric plasma (CAP)–mediated postsurgical cancer treatment, using a portable air-fed CAP (aCAP) device. The aCAP device we developed uses the local ambient air as the source gas to generate cold plasma discharge with only joule energy level electrical input, thus providing a device that is simple and highly tunable for a wide range of biomedical applications. We demonstrate that local aCAP treatment on residual tumor cells at the surgical cavities effectively induces cancer immunogenic cell death in situ and evokes strong T cell–mediated immune responses to combat the residual tumor cells. In both 4T1 breast tumor and B16F10 melanoma models, aCAP treatment after incomplete tumor resection contributes to inhibiting tumor growth and prolonging survival.


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Thuppul A., Collins A.L., Wright P.L., Uchizono N.M., Wirz R.E., “Mass Flux and Current Distributions of Electrospray Plumes,” J. Applied Physics 130, 103301 (2021); https://doi.org/10.1063/5.0056761

Performance and lifetime analysis of electrospray thrusters requires accurate knowledge of the mass and charge distributions of the plume. Mass flux and current density distributions were measured for a single capillary electrospray emitter using EMI-Im and found to be substantially different across a wide range of flow rates and emission voltages. Mass flux measurements yield an n~3 super-Gaussian profile across all flow rates and voltages, while current density measurements change shape from n~1.5-2.5 –super-Gaussian profiles monotonically with decreasing flow rate, where n=1 is Gaussian and higher n values correspond to increasingly more flat-top “super-Gaussian” profiles with steeper drop-off toward higher angles. For increasing flow rate, the mass flux profile grows while maintaining its shape, whereas the current density profile exhibits higher kurtosis, i.e., plumes that distribute proportionately more charge to higher angles. Additionally, higher extraction voltages exhibited tilted emission that led to highly off-axis plumes, ~10 degrees, for both mass flux and current density. Lifetime and performance assessments of electrospray thrusters must consider that mass flux and current density in the plume display different distribution shapes and trends in shape across changes in extraction voltage and flow rate.


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Ottaviano A., Thuppul A., Hayes J., Dodson C., Li G., Chen Z., and Wirz, R.E., “In-situ Microscopy of Ion-Inducted Sputter Erosion of a Featured Surface,” Review of Scientific Instruments, 92, 073701 (2021); https://doi.org/10.1063/5.0043002

A novel method for the in situ visualization and profilometry of a plasma-facing surface is demonstrated using a long-distance microscope. The technique provides valuable in situ monitoring of the microscopic temporal and morphological evolution of a material surface subject to plasma–surface interactions, such as ion-induced sputter erosion. Focus variation of image stacks enables height surface profilometry, which allows a depth of field beyond the limits associated with high magnification. As a demonstration of this capability, the erosion of a volumetrically featured aluminum foam is quantified during ion-bombardment in a low-temperature argon plasma where the electron temperature is ∼7 eV and the plasma is biased relative to the target surface such that ions impinge at ∼300 eV. Three-dimensional height maps are reconstructed from the images captured with a long-distance microscope with an x–y resolution of 3 × 3 μm² and a focus-variation resolution based on the motor step-size of 20 μm. The time-resolved height maps show a total surface recession of 730 μm and significant ligament thinning over the course of 330 min of plasma exposure. This technique can be used for developing plasma-facing components for a wide range of plasma devices for applications such as propulsion, manufacturing, hypersonics, and fusion.


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Li G., Wirz R.E., “Persistent Sputtering Yield Reduction in Plasma-Infused Foams,” Physical Review Letters, 126 (3), 035001 (2021); https://doi.org/10.1103/PhysRevLett.126.035001

Aluminum microfoams are found to exhibit persistent sputtering yield reductions of 40%–80% compared to a flat aluminum surface under 100 to 300 eV argon plasma bombardment. An analytical model reveals a strong dependency of the yield on the foam geometry and plasma sheath. For foam pore sizes near or larger than the sheath thickness, the plasma infuses the foam and transitions the plasma-surface interactions from superficial to volumetric phenomena. By defining a plasma infusion parameter, the sputtering behavior of foams is shown to be separated into the plasma-facing and plasma-infused regimes. While plasma infusion leads to a larger effective sputtering area, geometric recapture of ejected particles facilitates an overall reduction in yield. For a given level of plasma infusion, the reductions in normalized yield are more pronounced at lower ion energies since angular sputtering effects enable more effective geometric recapture of sputterants.


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Chen Z., Obenchain R., Wirz R.E., “Tiny cold atmospheric plasma jet for biomedical applications,” Featured Article Processes, 9(2), 249 (2021); https://doi.org/10.3390/pr9020249

Conventional plasma jets for biomedical applications tend to have several drawbacks, such as high voltages, high gas delivery, large plasma probe volume, and the formation of discharge within the organ. Therefore, it is challenging to employ these jets inside a living organism’s body. Thus, we developed a single-electrode tiny plasma jet and evaluated its use for clinical biomedical applications. We investigated the effect of voltage input and flow rate on the jet length and studied the physical parameters of the plasma jet, including discharge voltage, average gas and subject temperature, and optical emissions via spectroscopy (OES). The interactions between the tiny plasma jet and five subjects (de-ionized (DI) water, metal, cardboard, pork belly, and pork muscle) were studied at distances of 10 mm and 15 mm from the jet nozzle. The results showed that the tiny plasma jet caused no damage or burning of tissues, and the ROS/RNS (reactive oxygen/nitrogen species) intensity increased when the distance was lowered from 15 mm to 10 mm. These initial observations establish the tiny plasma jet device as a potentially useful tool in clinical biomedical applications.


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Wright P.L., Wirz R.E., “Multiplexed Electrospray Emission on a Porous Wedge,” Physics of Fluids 33, 012003 (2021); https://doi.org/10.1063/5.0030031

Linear porous wedge electrospray emitters exhibit a discrete number of emission sites that naturally form during operation. An analytical model is developed to examine the behavior and spacing of these emission sites via the pressure variation in the porous fluid flow associated with the flow focusing on each emission site, which is coupled with the local electric field. The solution for site spacing and current is informed by empirical results with support from electric field modeling and investigation of porous media parameters. Emission site currents of up to 500 nA and site spacings of roughly 50 µm–300 µm are predicted. Results from the model match well with experimental trends and provide further insights into the current and spacing of the discrete emission sites. These insights include the following: (1) for the investigated geometry, the total current can be estimated without taking into account the effects local to each emission site, (2) the wedge hydraulic resistance shows how the emitter output scales with emitter geometry and propellant properties, and (3) the emitted charge to mass ratio increases with the applied electric field. Last, we present a physical description of how specific charge increases with the restorative pressure from the reservoir.


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Samples S.A., Wirz R.E., “Parametric Analysis of High Delta-V CubeSat Missions with a Miniature Ion Thruster,” Journal of Spacecraft and Rockets, Vol. 58, No. 3, 2021; https://doi.org/10.2514/1.A34827

The increasing capabilities of the CubeSat platform have led to growing interest in performing more complex commercial and science missions with these miniature spacecraft. In particular, electric propulsion provides unprecedented mission Δ𝑉 and enables ambitious but low-cost Earth missions as well as lunar, asteroid, and interplanetary exploration. As a case study, a 6 U (where U represents a 10×10×10cm “unit”) CubeSat using the Miniature Xenon Ion Thruster was designed for a notional 3000m/s Δ𝑉 mission with a 2 kg, 1.6 U payload, resulting in a spacecraft wet mass of 11.9 kg and a burn duration of 15 months. This spacecraft is capable of up to 5.8km/s Δ𝑉 for a 0.5 U payload, as well as 2.1km/s for a 2 U payload. Parametric analyses with generalized electric thruster properties show that mission performance is sensitive to thruster 𝐼𝑠𝑝 and total efficiency 𝜂𝑇, with a 10% increase in efficiency resulting in a 16% decrease in burn time. This decrease in burn time is also possible by decreasing 𝐼𝑠𝑝, but it incurs mass and payload volume penalties. Parametric studies of neutralizer cathode properties show that neutralizer cathode selection is critical, and that such cathodes should be designed to require low power, low to zero flow rate, and long life for high-Δ𝑉 missions.


2020

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Chen Z., Garcia G., Arumugaswami V., Wirz R.E., “Cold atmospheric plasma for SARS-CoV-2 inactivation,” Featured Article Physics of Fluids 32, 111702 (2020); https://doi.org/10.1063/5.0031332

Syndrome coronavirus 2 (SARS-CoV-2) infectious virions are viable on various surfaces (e.g., plastic, metals, and cardboard) for several hours. This presents a transmission cycle for human infection that can be broken by developing new inactivation approaches. We employed an efficient cold atmospheric plasma (CAP) with argon feed gas to inactivate SARS-CoV-2 on various surfaces including plastic, metal, cardboard, basketball composite leather, football leather, and baseball leather. These results demonstrate the great potential of CAP as a safe and effective means to prevent virus transmission and infections for a wide range of surfaces that experience frequent human contact. Since this is the first-ever demonstration of cold plasma inactivation of SARS-CoV-2, it is a significant milestone in the prevention and treatment of coronavirus disease 2019 (COVID-19) and presents a new opportunity for the scientific, engineering, and medical communities.


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Peter L. Wright , Stephen A. Samples , Nolan M. Uchizono , and Richard E. Wirz, “Comment on “Jet propulsion by microwave air plasma in the atmosphere” [AIP Adv. 10, 055002 (2020)]”, AIP Advances 10, 099101 (2020); https://doi.org/10.1063/5.0013575

In this Comment, we analyze the performance of a microwave plasma device presented by Ye et al. [AIP Adv. 10, 055002 (2020)]. The efficiency analysis, using conservation of energy, shows that the methods used by the original authors predict up to 8000% device efficiency. Our analytical model is based on a control volume analysis of the original authors’ experimental setup and conditions, indicating that blocking the exit of the device yields stagnation pressure rather than jet pressure. The results from this analysis are consistent with the reported experimental data, demonstrating that the measured pressure using this method is internal chamber pressure and cannot be used to estimate thrust.


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Angelopoulos,…, R.E. Wirz, et al., “The ELFIN Mission,” Space Science Reviews (2020) 216:103 https://doi.org/10.1007/s11214-020-00721-7

The Electron Loss and Fields Investigation with a Spatio-Temporal Ambiguity-Resolving option (ELFIN-STAR, or heretoforth simply: ELFIN) mission comprises two identical 3-Unit (3U) CubeSats on a polar (∼93∘ inclination), nearly circular, low-Earth (∼450 km altitude) orbit. Launched on September 15, 2018, ELFIN is expected to have a >2.5 year lifetime. Its primary science objective is to resolve the mechanism of storm-time relativistic electron precipitation, for which electromagnetic ion cyclotron (EMIC) waves are a prime candidate. From its ionospheric vantage point, ELFIN uses its unique pitch-angle-resolving capability to determine whether measured relativistic electron pitch-angle and energy spectra within the loss cone bear the characteristic signatures of scattering by EMIC waves or whether such scattering may be due to other processes. Pairing identical ELFIN satellites with slowly-variable along-track separation allows disambiguation of spatial and temporal evolution of the precipitation over minutes-to-tens-of-minutes timescales, faster than the orbit period of a single low-altitude satellite (Torbit ∼ 90 min). Each satellite carries an energetic particle detector for electrons (EPDE) that measures 50 keV to 5 MeV electrons with E/E < 40% and a fluxgate magnetometer (FGM) on a ∼72 cm boom that measures magnetic field waves (e.g., EMIC waves) in the range from DC to 5 Hz Nyquist (nominally) with <0.3 nT/sqrt(Hz) noise at 1 Hz. The spinning satellites (Tspin 3 s) are equipped with magnetorquers (air coils) that permit spin-up or -down and reorientation maneuvers. Using those, the spin axis is placed normal to the orbit plane (nominally), allowing full pitch-angle resolution twice per spin. An energetic particle detector for ions (EPDI) measures 250 keV – 5 MeV ions, addressing secondary science. Funded initially by CalSpace and the University Nanosat Program, ELFIN was selected for flight with joint support from NSF and NASA between 2014 and 2018 and launched by the ELaNa XVIII program on a Delta II rocket (with IceSatII as the primary). Mission operations are currently funded by NASA. Working under experienced UCLA mentors, with advice from The Aerospace Corporation and NASA personnel, more than 250 undergraduates have matured the ELFIN implementation strategy; developed the instruments, satellite, and ground systems and operate the two satellites. ELFIN’s already high potential for cutting-edge science return is compounded by concurrent equatorial Heliophysics missions (THEMIS, Arase, Van Allen Probes, MMS) and ground stations. ELFIN’s integrated data analysis approach, rapid dissemination strategies via the SPace Environment Data Analysis System (SPEDAS), and data coordination with the Heliophysics/Geospace System Observatory (H/GSO) optimize science yield, enabling the widest community benefits. Several storm-time events have already been captured and are presented herein to demonstrate ELFIN’s data analysis methods and potential. These form the basis of on-going studies to resolve the primary mission science objective. Broad energy precipitation events, precipitation bands, and microbursts, clearly seen both at dawn and dusk, extend from tens of keV to >1 MeV. This broad energy range of precipitation indicates that multiple waves are providing scattering concurrently. Many observed events show significant backscattered fluxes, which in the past were hard to resolve by equatorial spacecraft or non-pitch-angle-resolving ionospheric missions. These observations suggest that the ionosphere plays a significant role in modifying magnetospheric electron fluxes and wave-particle interactions. Routine data captures starting in February 2020 and lasting for at least another year, approximately the remainder of the mission lifetime, are expected to provide a very rich dataset to address questions even beyond the primary mission science objective.


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Uchizono N.M., Collins A.L., Thuppul A., Wright P.L., Eckhardt D.Q., Ziemer J.K., Wirz R.E., “Emission modes in electrospray thrusters operating with high conductivity ionic liquids,” Aerospace, Special Issue: Electric Propulsion, 2020, 7(10), 141; https://doi.org/10.3390/aerospace7100141

Electrospray thruster life and mission performance are strongly influenced by grid impingement, the extent of which can be correlated with emission modes that occur at steady-state extraction voltages, and thruster command transients. Most notably, we experimentally observed skewed cone-jet emission during steady-state electrospray thruster operation, which leads to the definition of an additional grid impingement mechanism that we termed “tilted emission”. Long distance microscopy was used in conjunction with high speed videography to observe the emission site of an electrospray thruster operating with an ionic liquid propellant (EMI-Im). During steady-state thruster operation, no unsteady electrohydrodynamic emission modes were observed, though the conical meniscus exhibited steady off-axis tilt of up to 15°. Cone tilt angle was independent over a wide range of flow rates but proved strongly dependent on extraction voltage. For the geometry and propellant used, the optimal extraction voltage was near 1.6 kV. A second experiment characterized transient emission behavior by observing startup and shutdown of the thruster via flow or voltage. Three of the four possible startup and shutdown procedures transition to quiescence within ∼475 μs, with no observed unsteady modes. However, during voltage-induced thruster startup, unsteady electrohydrodynamic modes were observed.


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Magnusson J.M., Collins A.L., Wirz R.E., “Polyatomic Ion-Induced Electron Emission (IIEE) in Electrospray Thrusters”, Aerospace, Special Issue: Electric Propulsion, 2020, 7(11), 153, https://doi.org/10.3390/aerospace7110153

To better characterize the lifetime and performance of electrospray thrusters, electron emission due to electrode impingement by the propellant cation 1-ethyl-3-methylimidazolium (EMI+) has been evaluated with semi-empirical modeling techniques. Results demonstrate that electron emission due to grid impingement by EMI+ cations becomes significant once EMI+ attains a threshold velocity of ∼9×10⁵ cm s⁻¹. The mean secondary electron yield, 𝛾, exhibits strong linearity with respect to EMI+ velocity for typical electrospray operating regimes, and we present a simple linear fit equation corresponding to thruster potentials greater than 1 kV. The model chosen for our analysis was shown to be the most appropriate for molecular ion bombardments and is a useful tool in estimating IIEE yields in electrospray devices for molecular ion masses less than ∼1000 u and velocities greater than ∼10⁶ cm s⁻¹. Droplet-induced electron emission (DIEE) in electrospray thrusters was considered by treating a droplet as a macro-ion, with low charge-to-mass ratio, impacting a solid surface. This approach appears to oversimplify back-spray phenomena, meaning a more complex analysis is required. While semi-empirical models of IIEE, and the decades of solid state theory they are based upon, represent an invaluable advance in understanding secondary electron emission in electrospray devices, further progress would be gained by investigating the complex surfaces the electrodes acquire over their lifetimes and considering other possible emission processes.


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Thuppul A., Wright P.L., Collins A.L., Ziemer J.K., Wirz R.E., "Lifetime Considerations for Electrospray Thrusters", Aerospace, Special Issue: Electric Propulsion, 2020, 7(8), 108, https://doi.org/10.3390/aerospace7080108

Ionic liquid electrospray thrusters are capable of producing microNewton precision thrust at a high thrust–power ratio but have yet to demonstrate lifetimes that are suitable for most missions. Accumulation of propellant on the extractor and accelerator grids is thought to be the most significant life-limiting mechanism. In this study, we developed a life model to examine the effects of design features, operating conditions, and emission properties on the porous accelerator grid saturation time of a thruster operating in droplet emission mode. Characterizing a range of geometries and operating conditions revealed that modifying grid aperture radius and grid spacing by 3–7% can significantly improve thruster lifetime by 200–400%, though a need for explicit mass flux measurement was highlighted. Tolerance analysis showed that misalignment can result in 20–50% lifetime reduction. In addition, examining the impact of electron backstreaming showed that increasing aperture radius produces a significant increase in backstreaming current compared to changing grid spacing. A study of accelerator grid bias voltages revealed that applying a reasonably strong accelerator grid potential (in the order of a kV) can minimize backstreaming current to negligible levels for a range of geometries.


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Barde A., Nithyanandam K., Shinn M., Wirz R.E., "Sulfur Heat Transfer Behavior for Uniform and Non-uniform Thermal Charging of Horizontally-oriented Isochoric Thermal Energy Storage Systems", Intl. J. Heat Mass Transfer, 2020, 153, 119556, https://doi.org/10.1016/j.ijheatmasstransfer.2020.119556

Elemental sulfur is a low-cost, chemically stable thermal storage medium suitable for many medium to high temperature applications. In this study, we investigate the heat transfer behavior of sulfur, isochorically stored in a horizontally-oriented thermal storage element (steel tube) using experimental, analytical, and computational methods. The sulfur container was uniformly and non-uniformly heated along its axis from 50 to 600 °C to simulate the potential operating conditions for the full-scale thermal energy storage systems. The results of the study reveal distinct sulfur heat transfer mechanisms based on the temperature range and mode of thermal charging. For temperatures from 50 to 200 °C, the sulfur heat transfer behavior is governed by two primary mechanisms; 1) solid–liquid phase change, and 2) sulfur viscosity that varies strongly with temperature. From 200 to 600 °C, the buoyancy-driven natural convection is the dominant heat transfer mechanism and facilitates significantly high thermal charge rates. For axially non-uniform thermal charging, the axial temperature gradient induces natural convection along the axis that rapidly redistributes the thermal energy within the sulfur mass. Such axial convection has a strong impact on the thermal characteristics, including thermal charge/discharge rate and exergetic efficiency of the thermal storage systems. These observations and the high-fidelity computational model used in this study provide important means to identify the design parameters and operating conditions for which sulfur-based thermal energy storage (SulfurTES) systems will provide desirable thermal performance at a low thermal storage cost.


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Uchizono N.M., Samples S.A., Wirz R.E., "Tunable Reflectionless Absorption of Electromagnetic Waves in a Plasma–Metamaterial Composite Structure", Plasma Sources Sci. Technol., 2020, 29, 085009, https://doi.org/10.1088/1361-6595/aba489, pdf

We present the first experimental demonstration of a tunable reflectionless absorption resonance in a metamaterial integrated with a plasma discharge. A one-dimensional metamaterial structure excites transverse magnetic slow-wave modes known as 'spoof' surface plasmon polaritons. When interfaced with an argon plasma discharge, the metamaterial-induced 'spoof' plasmon mode is converted to a plasmon polariton mode confined to the plasma/dielectric interface. The reflectionless absorption band that manifests in the metamaterial's spectral response exhibits a dependency on the plasma's electron density that agrees well with theory.


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Samples S.A., Wirz R.E., "Development of the MiXI Thruster with the ARCH Discharge", Plasma Res. Express, 2020, 2, 025008, https://doi.org/10.1088/2516-1067/ab906d

The Miniature Xenon Ion (MiXI) thruster with the Axial Ring-Cusp Hybrid 'MiXI(ARCH)' discharge was developed and operated with beam extraction at 1 kV. The thruster achieved 59% cathode-free total discharge efficiency at 23.7 mA xenon beam current with filament cathodes and low temperature operation, corresponding to a discharge loss of 226 W/A and propellant utilization of 72%. Thruster efficiency was observed to increase with increasing flow rate and decrease with increasing temperature up to thermal steady state. At thermal steady state, the thruster anode reached ∼320 °C due to the thermal isolation of the thruster head. Reducing the discharge chamber aspect ratio from 0.5 to 0.4 increased thermal steady state efficiency from 46% to 57% but required slow ramping of beam voltage and was limited to stable operation to above 0.5 sccm discharge propellant flow. In contrast to the 3-ring cusp configuration, MiXI(3-Ring), the performance is generally higher but is not able to achieve lower thrust levels and requires more complex start-up for stable operation. An analytical single-cell model was developed and applied to investigate internal processes of the MiXI(ARCH) discharge. The model emulated the effect of increasing flow on performance, indicating that the dominant loss mechanism is plasma electron current to the anode, in contrast to the 3-Ring geometry, which is dominated by primary electron losses. This model also matched trends reported in previous works of strongly increasing electron temperature and primary density with propellant utilization. Through this effort, the MiXI thruster's highest achievable total efficiency has been increased, and several mechanisms for further improved efficiency have been identified.


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Jin K., Wirz R.E., "Sulfur Heat Transfer Behavior in a Vertically-Oriented and Nonuniformly-Heated Isochoric Thermal Energy Storage System", Applied Energy, 2020, Vol. 260, 114287, https://doi.org/10.1016/j.apenergy.2019.114287

Elemental sulfur thermal energy storage (SulfurTES) is a promising low-cost solution for many medium to high temperature (300–1200 °C) TES applications. Demonstrations of SulfurTES have shown that the heat transfer behavior of sulfur in isochoric tubes is critical to system thermal performance. Previous studies have elucidated and quantified the sulfur heat transfer rate for idealized uniform charge and discharge; however, nonuniform conditions are more likely to be encountered in practice and need to be understood. This paper uses experimental and computational efforts to investigate sulfur heat transfer as well as exergy and energy performance in vertically-oriented tubes for two nonuniform thermal charge scenarios: top-heating and bottom-heating. In comparison with uniform thermal charge, the top-heating causes significant thermal stratification of sulfur that helps the SulfurTES system achieve superior exergetic performance. In contrast, the bottom-heating causes rapid mixing between hot and cold sulfur resulting in high charge rates. Both nonuniform charge strategies could be utilized during the operation of the SulfurTES system to improve system performance as well as provide operational flexibility. Using the computational results, this article originally develops two simplified analytical procedures to estimate the energy and exergy performance of sulfur in tubes of different sizes under top- and bottom-heating. The current study provides significant qualitative and quantitative heat transfer descriptions and design bases for SulfurTES systems and encourages further investigations into the complicated thermal performance for other thermal storage applications.


2019

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Huerta C.E., Wirz R.E., “Ion-induced electron emission reduction via complex surface trapping,” AIP Advances, Vol. 9, Issue 12, (Editor’s Pick, Featured on Cover) 2019, https://doi.org/10.1063/1.5120519

abstract


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Chiu P.K., Roth-Johnson P.M., Wirz R.E., “Optimal structural design of biplane wind turbine blades,” Renewable Energy, Vol. 147, 2019, https://doi.org/10.1016/j.renene.2019.08.143

Biplane wind turbine blades have been shown to have improved structural performance, aerodynamic performance, and reduced aerodynamic loads compared to conventional blade designs. Here, the impact of these factors on blade mass is quantified for the first time. The objectives of this work are to quantify the mass of biplane wind turbine blades which have been designed for realistic loads, and to understand the mass-driving constraints for such blades. A numerical optimization approach is used to design the internal structure of biplane wind turbine blades, minimizing blade mass subject to a number of design requirements which are imposed as constraints. The mass reductions are significant, showing that the optimal biplane blades are more than 45% lighter than a similarly-optimized monoplane blade. This is primarily due to the improved resistance to flapwise deflection when compared to the monoplane blades, which allows for considerably less spar cap material to be used in the biplanes. Biplane blades are also shown to have improved resistance to edgewise fatigue damage, requiring less trailing edge reinforcement. Given such large mass reductions, some criticality is required, and the limitations of the present approach are discussed. The results of the optimization present strong evidence that biplane wind turbine blades may be an enabling concept for the next generation of lighter, larger, and more cost-effective wind turbine blades.


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Dodson C.A., Jorns B.A., Wirz R.E., “Measurements of ion velocity and wave propagation in a hollow cathode plume,” Plasma Source Science and Technology, 28, 065009, 2019, https://iopscience.iop.org/article/10.1088/1361-6595/ab1c48

The mechanism responsible for the production of energetic ions in the plume of hollow cathodes for electric propulsion is still an open issue. These ions are of concern to cathode and thruster lifetime, particularly for cathodes operating at high (>20 A) discharge currents. Recent theoretical and experimental investigations suggest that there is a correlation between ion energy gain and ion acoustic turbulence. In this paper we present measurements of the evolution of the ion velocity distribution function in the near plume of a 100 A-class hollow cathode, operated in a regime in which the dominant mode is ion acoustic turbulence. Ion flow and thermal properties were related to measurements of the background plasma, fluctuation spectra, and dispersion relations obtained from an array of Langmuir probes. We found ions to flow outward from the cathode and accelerate downstream, to supersonic speeds, approximately aligned with the acoustic wave group velocity vectors. The directions of the ion flow and wave propagation were similar for a range of discharge currents and mass flow rates in the jet region of the plume. One operating condition showed a significant temperature increase, also in the direction of acoustic wave propagation, corresponding to the highest wave energy condition. These results are interpreted in the context of ion acoustic turbulence as a contributing mechanism for ion energy gain.


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Jin K., Barde A., Nithyanandam K., Wirz R.E., “Sulfur heat transfer behavior in vertically-oriented isochoric thermal energy storage systems,” Applied Energy, Vol. 240, 2019, https://doi.org/10.1016/j.apenergy.2019.02.077 (featured in Advances in Engineering, https://advanceseng.com/sulfurtes-next-generation-thermal-energy-storage/)

Elemental sulfur is a promising medium for moderate to high-temperature thermal energy storage (TES) systems due to its low cost and excellent chemical stability up to very high temperatures (1200 °C). Previous studies show that vertically-oriented tubes of isochorically contained thermal storage media (i.e., supercritical CO2) can exhibit higher heat transfer rates than horizontal tubes. Storing thermal storage media in vertical tubes in a TES system also has some potential system-level advantages related to exergy capacity, operation and maintenance, and cost. This paper investigates the heat transfer behavior and performance of sulfur contained in vertically-oriented tubes between room temperature (25 °C) and 600 °C. Experimental and computational analyses show that the natural convection heat transfer behavior for sulfur in a vertically-oriented tube is strongly dependent on the sulfur viscosity, which varies greatly over the range of temperatures used in this study. Validated Nusselt number correlations for vertical tubes of lengths between 0.5 and 3 m and diameters between 5.5 and 21.2 cm are developed for use in parametric studies and designs. In comparison to the horizontally-oriented tube, the vertical tube can have better heat transfer performance with some ranges of tube length and diameter. Therefore, the selection of the tube orientation strongly depends on the tube dimensions and application needs. The results from the current study provide important quantitative and qualitative design bases for sulfur-based TES (SulfurTES) systems.


2018

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Dankongkakul B., Wirz R.E., "Axial Ring-Cusp Hybrid (ARCH) Plasma Discharge Design: An Approach to Highly Efficient Miniature-Scale Ion Sources", Plasma Sources Science and Technology, 2018, https://iopscience.iop.org/article/10.1088/1361-6595/aae63c

The miniaturization of conventional direct-current ion sources is predominantly restricted by efficiency limitations associated with the increased surface area-to-volume ratio of smaller-scale discharge chambers—reducing the effective confinement length of the high-energy 'primary' electrons that is necessary for efficient plasma generation. The Axial Ring-Cusp Hybrid (ARCH) plasma discharge addresses this scaling limitation by using a new approach that combines magnetic and electrostatic confinement to decouple the primary and plasma electrons loss mechanisms. Simulated ion thruster performance measurements show that the ARCH discharge may be capable of achieving a discharge loss and a propellant mass utilization of 175 eV/ion and 0.87, respectively. These estimates are supported by full internal maps of the plasma properties, including the electron energy distribution function, inside the discharge chamber. The measurements show highly effective confinement of the primary electrons, high average plasma electron temperatures of ∼5 eV, and low plasma sheath potential relative to the anode—attributes generally found only in efficient conventional-scale discharges with good overall plasma confinement. As such, the new ARCH discharge design approach may allow miniature ion thrusters to achieve the performance and efficiency levels similar to those of highly efficient conventional ion thrusters.


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Patino M.I., Wirz R.E., Raitses Y., Koel B.E., "Angular, Temperature, and Impurity Effects on Secondary Electron Emission from Ni(110)", Journal of Applied Physics, Editor’s Pick, 2018, https://doi.org/10.1063/1.5025344

The secondary electron emission from a temperature-controlled Ni(110) sample was examined for 50–1500 eV electrons impacting at 0°–35°, 50°, and 78°. Measurements showed a non-cosine dependence on an electron incidence angle: the yield has a maximum at 0°, minima at ±12°, and increases at larger angles up to 35°. This trend in angular dependence is characteristic of single crystal materials and is due to increased secondary electron generation when primary electrons are directed along a close-packed direction. For example, compared to polycrystalline nickel, the yield for Ni(110) from primary electrons at 0° (i.e., along the [110] direction) is up to 36% larger. Additionally, secondary electron yields are highly sensitive to incident electron energy (most notably between 0 and 500 eV) and to the presence of adsorbed carbon monoxide [with an up to 25% decrease compared to clean Ni(110)]. However, yields are independent of sample temperature between 300 and 600 K and of exposure to deuterium ions leading to formation of subsurface hydrogen. These results reaffirm the unique secondary electron emission properties of single crystals materials and highlight the importance of crystal orientation. Results are important for plasma-enhanced chemistry applications that utilize Ni(110) catalysts, since larger secondary electron emission may facilitate reactions of adsorbed species.


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Dodson C.A., Perez-Grande D., Jorns B.A., Goebel D.M., Wirz R.E., "Ion Heating Measurements on the Centerline of a High-Current Hollow Cathode Plume", Journal of Propulsion and Power, 2018, https://doi.org/10.2514/1.B36788

An experimental investigation into the correlation between ion acoustic turbulence (IAT) and anomalous ion heating in the plume of a 100 A-class LaB6 hollow cathode is presented. Laser-induced fluorescence is employed to measure the ion velocity distribution function, and a translating ion saturation probe is used to quantify the spatial dependence of the IAT wave energy. It is found that over a range of flow rates and operating currents both the ion temperature and IAT energy increase downstream of the cathode in qualitatively similar ways. Both parameters also are shown to be impacted by operating conditions: the IAT energy and ion temperature decrease at higher flow rates and lower discharge currents. It is shown that the ratio between ion temperature and wave energy is related by a scaling parameter that depends on the background plasma parameters, and this relation is examined in the context of previous analytical work on IAT-induced ion heating.


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Patino M.I., Wirz R.E., "Characterization of Xenon Ion and Neutral Interactions in a Well-Characterized Experiment", Physics of Plasma, Vol. 25, 2018, 062108, https://doi.org/10.1063/1.5030464

Interactions between fast ions and slow neutral atoms are commonly dominated by charge-exchange and momentum-exchange collisions, which are important to understanding and simulating the performance and behavior of many plasma devices. To investigate these interactions, this work developed a simple, well-characterized experiment that accurately measures the behavior of high energy xenon ions incident on a background of xenon neutral atoms. By using well-defined operating conditions and a simple geometry, these results serve as canonical data for the development and validation of plasma models and models of neutral beam sources that need to ensure accurate treatment of angular scattering distributions of charge-exchange and momentum-exchange ions and neutrals. The energies used in this study are relevant for electric propulsion devices ∼1.5 keV and can be used to improve models of ion-neutral interactions in the plume. By comparing these results to both analytical and computational models of ion-neutral interactions, we discovered the importance of (1) accurately treating the differential cross-sections for momentum-exchange and charge-exchange collisions over a large range of neutral background pressures and (2) properly considering commonly overlooked interactions, such as ion-induced electron emission from nearby surfaces and neutral-neutral ionization collisions.


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Barde A., Jin K., Shinn M., Nithyanandam K., Wirz R.E., "Demonstration of a Low Cost, High Temperature Elemental Sulfur Thermal Battery", Applied Thermal Engineering, Vol. 137, June 2018, pp. 259-267, https://doi.org/10.1016/j.applthermaleng.2018.02.094

Elemental sulfur is a low-cost energy storage media suitable for many medium to high temperature applications, including trough and tower concentrated solar power (CSP) and combined heat and power (CHP) systems. In this study, we have demonstrated the viability of an elemental sulfur thermal energy storage (SulfurTES) system using a laboratory-scale thermal battery. The SulfurTES battery design uses a shell-and-tube thermal battery configuration, wherein stationary elemental sulfur is isochorically stored in multiple stainless steel tubes and a heat transfer fluid (air) is passed over them through the surrounding shell. The safe and reliable operation was demonstrated for twelve thermal charge–discharge cycles in the temperature range of 200–600 °C, during which the SulfurTES battery stored up to 7.6 kW h of thermal energy with volumetric energy density range up to 255 kW h/m3. Furthermore, the SulfurTES battery is operated in a hybrid thermal charging mode to demonstrate its ability to store surplus electrical energy. The present study establishes the feasibility of SulfurTES as a concept that could provide attractive system cost and volumetric energy density for a wide range of thermal energy storage applications.


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Lakeh R.B., Wirz R.E., Kavehpour P., Lavine A.S., "A Dimensionless Model for Transient Turbulent Natural Convection in Isochoric Vertical Thermal Energy Storage Tubes", Journal of Thermal Science and Engineering Applications, Vol. 10(3), June 2018, 034501, https://doi.org/10.1115/1.4038587

In this study, turbulent natural convection heat transfer during the charge cycle of an isochoric vertically oriented thermal energy storage (TES) tube is studied computationally and analytically. The storage fluids considered in this study (supercritical CO2 and liquid toluene) cover a wide range of Rayleigh numbers. The volume of the storage tube is constant and the thermal storage happens in an isochoric process. A computational model was utilized to study turbulent natural convection during the charge cycle. The computational results were further utilized to develop a conceptual and dimensionless model that views the thermal storage process as a hot boundary layer that rises along the tube wall and falls in the center to replace the cold fluid in the core. The dimensionless model predicts that the dimensionless mean temperature of the storage fluid and average Nusselt number of natural convection are functions of L/D ratio, Rayleigh number, and Fourier number that are combined to form a buoyancy-Fourier number.


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Huerta C.E., Patino M.I., Wirz R.E., "Secondary Electron Emission from Textured Surfaces", Journal of Physics D: Applied Physics, Vol. 51(14), March 2018, 145202, https://doi.org/10.1088/1361-6463/aab1ac

In this work, a Monte Carlo model is used to investigate electron induced secondary electron emission for varying effects of complex surfaces by using simple geometric constructs. Geometries used in the model include: vertical fibers for velvet-like surfaces, tapered pillars for carpet-like surfaces, and a cage-like configuration of interlaced horizontal and vertical fibers for nano-structured fuzz. The model accurately captures the secondary electron emission yield dependence on incidence angle. The model shows that unlike other structured surfaces previously studied, tungsten fuzz exhibits secondary electron emission yield that is independent of primary electron incidence angle, due to the prevalence of horizontally-oriented fibers in the fuzz geometry. This is confirmed with new data presented herein of the secondary electron emission yield of tungsten fuzz at incidence angles from 0–60°.


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Nithyanandam K., Barde A., Lakeh R.B., Wirz R.E., "Charge and Discharge Behavior of Elemental Sulfur in Isochoric High Temperature Thermal Energy Storage Systems", Applied Energy, Vol. 214, March 2018, pp. 166-177, https://doi.org/10.1016/j.apenergy.2017.12.121

Thermal energy storage with elemental sulfur is a low-cost alternative to molten salts for many medium to high-temperature energy applications (200–600 °C). In this effort, by examining elemental sulfur stored isochorically inside isolated pipes, we find that sulfur provides attractive charge/discharge performance since it operates in the liquid-vapor regime at the temperatures relevant to many important applications, such as combined heat and power (CHP) plants and concentrating solar power (CSP) plants with advanced power cycle systems. The isolated pipe configuration is relevant to shell-and-tube thermal battery applications where the heat transfer fluid flows over the storage pipes through the shell. We analyze the transient charge and discharge behavior of sulfur inside the pipes using detailed computational modeling of the complex conjugate heat transfer and fluid flow phenomena. The computational model is validated against experiments of a single tube with well-defined temperature boundary conditions and internal temperature measurements. The model results evaluate the influence of pipe diameter on charge and discharge times, heat transfer rate, and Nusselt number due to buoyancy driven convection currents. Depending on the Rayleigh number (pipe diameter), the average Nusselt number obtained for discharge is 3–14 times higher than proposed solid-liquid phase change technologies based on molten salt, which are limited in their performance due to conduction based solidification and low thermal conductivity. The results show competing trade-offs between increase in heat transfer coefficient, thermal energy stored in sulfur, and increase in charge and discharge time with increase in pipe diameter. A preferred pipe diameter can be determined for target applications based on their requirements and these competing trade-offs. A validated fundamental correlation for Nusselt number as a function of Rayleigh number for charge and discharge is developed that can be used to design the sulfur-based thermal storage system for transient operation.


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Shinn M., Nithyanandam K., Barde A., Wirz R.E., "Sulfur-based Thermal Energy Storage System Using Intermodal Containment: Design and Performance Analysis", Applied Thermal Engineering, Vol. 128, January 2018, pp. 1009-1021, https://doi.org/10.1016/j.applthermaleng.2017.08.167

Thermal energy storage (TES) is an important energy storage technology that can be coupled to intermittent energy sources to improve system dispatchability. Elemental sulfur is a promising candidate storage fluid for high temperature TES systems due to its high energy density, moderate vapor pressure, high thermal stability, and low cost. This study uses a transient, two-dimensional numerical model to investigate the design and performance of a thermal energy storage (TES) system that uses sulfur stored isochorically in an intermodal shell and tube thermal battery configuration. Parametric analyses of key design and operating parameters show that there is a preferred tube diameter based on the competing influence of system-level energy storage utilization, exergetic efficiency, and cost. The results show that designs with smaller tube dimensions in the range of 2″ NPS to 4″ NPS provide exergetic efficiencies close to 95% while tube dimensions in the range of 4″ NPS to 8″ NPS meet the Department of Energy cost target of $15/kWh with costs being as low as $8.41/kWh. Finally, a table of preferred designs that meet the DOE cost goals is presented to help guide future design and experimentation efforts.


2017

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Dankongkakul B., Wirz R.E., "Miniature Ion Thruster Ring-Cusp Discharge Performance and Behavior", Journal of Applied Physics, Vol. 122, 023208, 2017, https://doi.org/10.1063/1.4995638

Miniature ion thrusters are an attractive option for a wide range of space missions due to their low power levels and high specific impulse. Thrusters using ring-cusp plasma discharges promise the highest performance, but are still limited by the challenges of efficiently maintaining a plasma discharge at such small scales (typically 1–3 cm diameter). This effort significantly advances the understanding of miniature-scale plasma discharges by comparing the performance and xenon plasma confinement behavior for 3-ring, 4-ring, and 5-ring cusp by using the 3 cm Miniature Xenon Ion thruster as a modifiable platform. By measuring and comparing the plasma and electron energy distribution maps throughout the discharge, we find that miniature ring-cusp plasma behavior is dominated by the high magnetic fields from the cusps; this can lead to high loss rates of high-energy primary electrons to the anode walls. However, the primary electron confinement was shown to considerably improve by imposing an axial magnetic field or by using cathode terminating cusps, which led to increases in the discharge efficiency of up to 50%. Even though these design modifications still present some challenges, they show promise to bypassing what were previously seen as inherent limitations to ring-cusp discharge efficiency at miniature scales.


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Conversano R.W., Goebel D.M., Hofer R.R., Mikellides I.G., Wirz R.E., "Performance Analysis of a Low-Power Magnetically Shielded Hall Thruster: Experiments", Journal of Propulsion and Power, Vol. 33(4), pp. 975-983, Aug. 2017, https://doi.org/10.2514/1.B36230

The successful application of a fully shielding magnetic field topology in a low-power Hall thruster is demonstrated through the testing of the MaSMi-60 Hall thruster (an improved variant of the original Magnetically Shielded Miniature Hall thruster). The device was operated at discharge powers from 160 to 750 W at discharge voltages ranging from 200 to 400 V. Several techniques were used to determine the effectiveness of magnetic shielding achieved by the MaSMi-60 and to estimate the reduction in discharge channel erosion rate enabled by the shielding field topology. This ultimately suggested an improvement in discharge channel life by a factor of at least 10 times, and likely greater than 100 times, when compared to unshielded devices. Thruster performance, measured both directly by a thrust stand and indirectly by plume diagnostics, was lower than expected when compared to results from high-power magnetically shielded Hall thrusters. However, the plume diagnostic measurements enabled the identification of the primary causes for the MaSMi-60’s moderate performance.


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Conversano R.W., Goebel D. M., Mikellides I.G., Hofer R.R., Wirz R.E., "Performance Analysis of a Low-Power Magnetically Shielded Hall Thruster: Computational Modeling", Journal of Propulsion and Power, Vol. 33(4), pp. 992-1001, Aug. 2017, https://doi.org/10.2514/1.B36231

The applicability of a fully shielding magnetic field topology to a low-power xenon Hall thruster was demonstrated through testing of the MaSMi-60. Although the discharge channel lifetime was significantly increased, performance testing of the device revealed a peak anode efficiency of under 30%, which was lower than expected given the available data on high-power magnetically shielded Hall thrusters. Experimental measurements in a vacuum facility with operating pressures of <5×106torr suggest that the MaSMi-60’s current utilization, mass utilization, and beam divergence efficiencies were the key contributors to its moderate performance. To better understand the physics causing these performance deficiencies, a computational analysis including 2D plasma modeling of the MaSMi-60 was conducted. Results from the 2D numerical models confirmed that the MaSMi-60 achieved the parameters necessary for magnetic shielding. The physics governing the low mass utilization, current utilization, and beam divergence efficiencies were then identified and described using the computational model results. The low mass utilization is attributed to a long ionization mean free path in the discharge channel caused by the predominantly axial trajectory of the injected propellant. Insufficient magnetic field strength enabling excessive electron current to the anode was the primary cause for the poor current utilization. Lastly, the high beam divergence was due to an overly shielding magnetic field topology that promoted high-energy ions to be accelerated far off the thruster’s axis.


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Jorns B.A., Dodson C., Goebel D.M., Wirz R.E., "Propagation of Ion Acoustic Wave Energy in the Plume of a High-Current LaB6 Hollow Cathode", Physical Review E, Vol. 96, 023208, Aug. 2017, https://doi.org/10.1103/PhysRevE.96.023208

A frequency-averaged quasilinear model is derived and experimentally validated for the evolution of ion acoustic turbulence (IAT) along the centerline of a 100-A class, LaB6 hollow cathode. Probe-based diagnostics and a laser induced fluorescence system are employed to measure the properties of both the turbulence and the background plasma parameters as they vary spatially in the cathode plume. It is shown that for the three discharge currents investigated, 100 A, 130 A, and 160 A, the spatial growth of the total energy density of the IAT in the near field of the cathode plume is exponential and agrees quantitatively with the predicted growth rates from the quasilinear formulation. However, in the downstream region of the cathode plume, the growth of IAT energy saturates at a level that is commensurate with the Sagdeev limit. The experimental validation of the quasilinear model for IAT growth and its limitations are discussed in the context of numerical efforts to describe self-consistently the plasma processes in the hollow cathode plume.


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Li G.Z., Matlock T.S., Goebel D.M., Dodson C.A., Matthes C.S., Ghoniem N.M., Wirz R.E., "In situ plasma sputtering and angular distribution measurements for structured molybdenum surfaces", Plasma Sources Sci. Technol., Vol. 26, 065002, April 2017, https://doi.org/10.1088/1361-6595/aa6a7d

We present in situ sputtering yield measurements of the time-dependent erosion of flat and micro-architectured molybdenum samples in a plasma environment. The measurements are performed using the plasma interactions (Pi) Facility at UCLA, which focuses a magnetized hollow cathode plasma to a material target with an exposure diameter of approximately 1.5 cm. During plasma exposure, a scanning quartz crystal microbalance (QCM) provides angular sputtering profiles that are integrated to estimate the total sputtering yield. This technique is validated to within the scatter of previous experimental data for a planar molybdenum target exposed to argon ion energies from 100 to 300 eV. The QCM is then used to obtain in situ measurements during a 17 h exposure of a micro-architectured-surface molybdenum sample to 300 eV incident argon ions. The time-dependent angular sputtering profile is shown to deviate from classical planar profiles, demonstrating the unique temporal and spatial sputtering effects of micro-architectured materials. Notably, the sputtering yield for the micro-architectured sample is initially much less than that for planar molybdenum, but then gradually asymptotes to the value for planar molybdenum after approximately 10 h as the surface features are eroded away.


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Matthes C.S., Ghoniem N.M., Li G.Z., Matlock T.S., Goebel D.M., Dodson C.A., Wirz R.E., "Fluence-Dependent Sputtering Yield of Micro-architectured Materials", Applied Surface Science, Vol. 407, pp. 223-235, June 2017, https://doi.org/10.1016/j.apsusc.2017.02.140

We present an experimental examination of the relationship between the surface morphology of Mo and its instantaneous sputtering rate as function of low-energy plasma ion fluence. We quantify the dynamic evolution of nano/micro features of surfaces with built-in architecture, and the corresponding variation in the sputtering yield. Ballistic deposition of sputtered atoms as a result of geometric re-trapping is observed, and re-growth of surface layers is confirmed. This provides a self-healing mechanism of micro-architectured surfaces during plasma exposure. A variety of material characterization techniques are used to show that the sputtering yield is not a fundamental property, but that it is quantitatively related to the initial surface architecture and to its subsequent evolution. The sputtering yield of textured molybdenum samples exposed to 300 eV Ar plasma is roughly 1/2 of the corresponding value for flat samples, and increases with ion fluence. Mo samples exhibited a sputtering yield initially as low as 0.22 ± 5%, converging to 0.4 ± 5% at high fluence. The sputtering yield exhibits a transient behavior as function of the integrated ion fluence, reaching a steady-state value that is independent of initial surface conditions. A phenomenological model is proposed to explain the observed transient sputtering phenomenon, and to show that the saturation fluence is solely determined by the initial surface roughness.


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Rivera D., Wirz R.E., Ghoniem N.M., "Experimental measurements of surface damage and residual stresses in micro-engineered plasma facing materials", Journal of Nuclear Materials, Vol. 486, pp. 111-121, April 2017, http://dx.doi.org/10.1016%2Fj.jnucmat.2016.12.035

The thermomechanical damage and residual stresses in plasma-facing materials operating at high heat flux are experimentally investigated. Materials with micro-surfaces are found to be more resilient, when exposed to cyclic high heat flux generated by an arc-jet plasma. An experimental facility, dedicated to High Energy Flux Testing (HEFTY), is developed for testing cyclic heat flux in excess of 10 MW/m2. We show that plastic deformation and subsequent fracture of the surface can be controlled by sample cooling. We demonstrate that W surfaces with micro-pillar type surface architecture have significantly reduced residual thermal stresses after plasma exposure, as compared to those with flat surfaces. X-ray diffraction (XRD) spectra of the W-(110) peak reveal that broadening of the Full Width at Half Maximum (FWHM) for micro-engineered samples is substantially smaller than corresponding flat surfaces. Spectral shifts of XRD signals indicate that residual stresses due to plasma exposure of micro-engineered surfaces build up in the first few cycles of exposure. Subsequent cyclic plasma heat loading is shown to anneal out most of the built-up residual stresses in micro-engineered surfaces. These findings are consistent with relaxation of residual thermal stresses in surfaces with micro-engineered features. The initial residual stress state of highly polished flat W samples is compressive (-1.3 GPa). After exposure to 50 plasma cycles, the surface stress relaxes to −1.0 GPa. Micro-engineered samples exposed to the same thermal cycling show that the initial residual stress state is compressive at (-250 MPa), and remains largely unchanged after plasma exposure.


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Patino M.I., Raitses Y., Wirz R.E., "Secondary Electron Emission from Plasma-Generated Nanostructured Tungsten Fuzz", Applied Physics Letters, Vol. 109, 201602, Nov. 2016, https://doi.org/10.1063/1.4967830

Recently, several researchers [e.g., Yang et al., Sci. Rep. 5, 10959 (2015)] have shown that tungsten fuzz can grow on a hot tungsten surface under bombardment by energetic helium ions in different plasma discharges and applications, including magnetic fusion devices with plasma facing tungsten components. This work reports the direct measurements of the total effective secondary electron emission (SEE) from tungsten fuzz. Using dedicated material surface diagnostics and in-situ characterization, we find two important results: (1) SEE values for tungsten fuzz are 40%–63% lower than for smooth tungsten and (2) the SEE values for tungsten fuzz are independent of the angle of the incident electron. The reduction in SEE from tungsten fuzz is most pronounced at high incident angles, which has important implications for many plasma devices since in a negative-going sheath the potential structure leads to relatively high incident angles for the electrons at the plasma confining walls. Overall, low SEE will create a relatively higher sheath potential difference that reduces plasma electron energy loss to the confining wall. Thus, the presence or self-generation in a plasma of a low SEE surface such as tungsten fuzz can be desirable for improved performance of many plasma devices.


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Huerta C.E., Matlock T.S., Wirz R.E., "View Factor Modeling of Sputter-Deposition on Micron-Scale-Architectured Surfaces Exposed to Plasma", Journal of Applied Physics, Vol. 119, 113303, March 2016, https://aip.scitation.org/doi/10.1063/1.4944035

The sputter-deposition on surfaces exposed to plasma plays an important role in the erosion behavior and overall performance of a wide range of plasma devices. Plasma models in the low density, low energy plasma regime typically neglect micron-scale surface feature effects on the net sputter yield and erosion rate. The model discussed in this paper captures such surface architecture effects via a computationally efficient view factor model. The model compares well with experimental measurements of argon ion sputter yield from a nickel surface with a triangle wave geometry with peak heights in the hundreds of microns range. Further analysis with the model shows that increasing the surface pitch angle beyond about 45° can lead to significant decreases in the normalized net sputter yield for all simulated ion incident energies (i.e., 75, 100, 200, and 400 eV) for both smooth and roughened surfaces. At higher incident energies, smooth triangular surfaces exhibit a nonmonotonic trend in the normalized net sputter yield with surface pitch angle with a maximum yield above unity over a range of intermediate angles. The resulting increased erosion rate occurs because increased sputter yield due to the local ion incidence angle outweighs increased deposition due to the sputterant angular distribution. The model also compares well with experimentally observed radial expansion of protuberances (measuring tens of microns) in a nano-rod field exposed to an argon beam. The model captures the coalescence of sputterants at the protuberance sites and accurately illustrates the structure's expansion due to deposition from surrounding sputtering surfaces; these capabilities will be used for future studies into more complex surface architectures.


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Patino M.I., Raitses Y., Koel B.E., Wirz R.E., "Analysis of Secondary Electron Emission for Conducting Materials Using 4-grid LEED/AES Optics", Journal of Physics D: Applied Physics, Vol. 48, 195204, April 2015, https://doi.org/10.1088/0022-3727/48/19/195204

A facility utilizing 4-grid optics for LEED/AES (low energy electron diffraction/Auger electron spectroscopy) was developed to measure the total secondary electron yield and secondary electron energy distribution function for conducting materials. The facility and experimental procedure were validated with measurements of 50–500 eV primary electrons impacting graphite. The total yield was calculated from measurements of the secondary electron current (i) from the sample and (ii) from the collection assembly, by biasing each surface. Secondary electron yield results from both methods agreed well with each other and were within the spread of previous results for the total yield from graphite. Additionally, measurements of the energy distribution function of secondary electrons from graphite are provided for a wider range of incident electron energies. These results can be used in modeling plasma-wall interactions in plasmas bounded by graphite walls, such as are found in plasma thrusters, and divertors and limiters of magnetic fusion devices.


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Lakeh R.B., Lavine A.S., Kavehpour H.P., Wirz R.E., "Study of Turbulent Natural Convection in Vertical Storage Tubes for Supercritical Thermal Energy Storage", Numerical Heat Transfer, Part A, Vol. 67(2), pp. 119-139, Oct. 2014, https://doi.org/10.1080/10407782.2014.923224

Heat transfer to storage fluid is a critical subject for any thermal energy storage system. The poor thermal conductivity of the storage medium may lead to insufficient heat transfer and may impair the functionality of the system. Supercritical thermal energy storage systems benefit from turbulent natural convection that dominates the heat transfer mechanism and compensates for the low thermal conductivity of the storage fluids. A computational model is validated and adopted to study the buoyancy-driven flow in vertical storage tubes and the effect of the aspect ratio of the vertical storage tubes on the charge time is investigated.


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Tse L.A., Lavine A.S., Lakeh R.B., Wirz R.E., "Exergetic Optimization and Performance Evaluation of Multi-Phase Thermal Energy Storage Systems", Solar Energy, Vol. 122, pp. 396–408, Dec. 2015, https://doi.org/10.1016/j.solener.2015.08.026

This study outlines a methodology for modeling and optimizing multi-phase thermal energy storage systems for solar thermal power plant (STPP) operation by incorporating energy and exergy analyses to a TES system employing a storage medium that can undergo multi-phase operation during the charging and discharging period. First, a numerical model is developed to investigate the transient thermodynamic and heat transfer characteristics of the storage system by coupling conservation of energy with an equation of state to model the spatial and temporal variations in fluid properties during the entire working cycle of the TES tank. Second, parametric studies are conducted to determine the impact of key design parameters on both energy and exergy efficiencies. The optimal values must balance exergy destroyed due to heat transfer and exergy destroyed due to pressure losses, which have competing effects. Optimization is utilized to determine parameter values within a feasible design window, which leads to a maximum exergetic efficiency of 87%.


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Dankongkakul B., Araki S. J., Wirz R.E., "Magnetic Field Structure Influence on Primary Electron Cusp Losses for Micro-scale Discharges", Physics of Plasmas, featured on cover, Vol. 21, 043506, April 2014, https://doi.org/10.1063/1.4871724

An experimental effort was used to examine the primary electron loss behavior for micro-scale (⁠ diameter) discharges. The experiment uses an electron flood gun source and an axially aligned arrangement of ring-cusps to guide the electrons to a downstream point cusp. Measurements of the electron current collected at the point cusp show an unexpectedly complex loss pattern with azimuthally periodic structures. Additionally, in contrast to conventional theory for cusp losses, the overall radii of the measured collection areas are over an order of magnitude larger than the electron gyroradius. Comparing these results to Monte Carlo particle tracking simulations and a simplified analytical analysis shows that azimuthal asymmetries of the magnetic field far upstream of the collection surface can substantially affect the electron loss structure and overall loss area.


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Conversano R., Goebel D.M., Hofer R.R., Matlock T.S., Wirz R.E., "Development and Initial Testing of a Magnetically Shielded Miniature Hall Thruster", IEEE Transactions on Plasma Science, Vol. 43(1), pp. 103-117, May 2014, https://doi.org/10.1109/TPS.2014.2321107

The scaling of magnetically shielded Hall thrusters to low power is investigated through the development and fabrication of a 4-cm Hall thruster. During initial testing, the magnetically shielded miniature Hall thruster was operated at 275 V discharge voltage and 325-W discharge power. Inspection of the channel walls after testing suggests that the outer discharge channel wall was successfully shielded from high-energy ion erosion while the inner channel wall showed evidence of weaker shielding, likely due to magnetic circuit saturation. Scanning planar probe measurements taken at two locations downstream of the thruster face provided ion current density profiles. The ion current calculated by integrating these data was 1.04 A with a plume divergence half-angle of 30°. Swept retarding potential analyzer measurements taken 80-cm axially downstream of the thruster measured the most probable ion voltage to be 252 V. The total thruster efficiency was calculated from probe measurements to be 43% (anode efficiency of 59%) corresponding to a thrust of 19 mN at a specific impulse of 1870 s. Discharge channel erosion rates were found to be approximately three orders of magnitude less than unshielded Hall thrusters, suggesting the potential for a significant increase in operational life.


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Bran-Anleu G., Lavine A.S., Wirz R.E., Kavehpour H.P., "Algorithm to Optimize Transient Hot-Wire Thermal Property Measurement", Review of Scientific Instruments, Vol. 85, 045105, April 2014, https://doi.org/10.1063/1.4870275

The transient hot-wire method has been widely used to measure the thermal conductivity of fluids. The ideal working equation is based on the solution of the transient heat conduction equation for an infinite linear heat source assuming no natural convection or thermal end effects. In practice, the assumptions inherent in the model are only valid for a portion of the measurement time. In this study, an algorithm was developed to automatically select the proper data range from a transient hot-wire experiment. Numerical simulations of the experiment were used in order to validate the algorithm. The experimental results show that the developed algorithm can be used to improve the accuracy of thermal conductivity measurements.


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Matlock T.S., Goebel D.M., Conversano R., Wirz R.E., "A DC Plasma Source for Plasma–Material Interaction Experiments", Plasma Sources Science and Technology, Vol. 23, 025014, March 2014, https://doi.org/10.1088/0963-0252/23/2/025014

A new device has been constructed for the investigation of interactions between engineered materials and a plasma in regimes relevant to electric propulsion and pulsed power devices. A linear plasma source, consisting of a hollow cathode, cylindrical anode, and axial magnetic field, delivers a 3 cm diameter beam to a biased target 70 cm away. The ion energy impacting the surface is controlled by biasing the sample from 0 to 500 V below the local plasma potential. This paper discusses the major aspects of the plasma source design and presents measurements of the plasma parameters achieved to date on argon and xenon. Experiments show that splitting the gas injection between the hollow cathode and the anode region provides control of the discharge voltage to minimize cathode sputtering while providing ion fluxes to the target in excess of 1021 m−2 s−1. Sputtering rate measurements on a non-textured molybdenum sample show close agreement with those established in the literature.


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Araki S.J., Wirz R.E., "Cell-centered Particle Weighting Algorithm for PIC Simulations in a Non-uniform 2D Axisymmetric Mesh", Journal of Computational Physics, Vol. 272, pp. 218–226, Sept. 2014, https://doi.org/10.1016/j.jcp.2014.04.037

Standard area weighting methods for particle-in-cell simulations result in systematic errors on particle densities for a non-uniform mesh in cylindrical coordinates. These errors can be significantly reduced by using weighted cell volumes for density calculations. A detailed description on the corrected volume calculations and cell-centered weighting algorithm in a non-uniform mesh is provided. The simple formulas for the corrected volume can be used for any type of quadrilateral and/or triangular mesh in cylindrical coordinates. Density errors arising from the cell-centered weighting algorithm are computed for radial density profiles of uniform, linearly decreasing, and Bessel function in an adaptive Cartesian mesh and an unstructured mesh. For all the density profiles, it is shown that the weighting algorithm provides a significant improvement for density calculations. However, relatively large density errors may persist at outermost cells for monotonically decreasing density profiles. A further analysis has been performed to investigate the effect of the density errors in potential calculations, and it is shown that the error at the outermost cell does not propagate into the potential solution for the density profiles investigated.


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Roth-Johnson P., Wirz R.E., "Structural Design of Spars for 100-m Biplane Wind Turbine Blades", Renewable Energy, Vol. 71, pp. 133-155, Nov. 2014, https://doi.org/10.1016/j.renene.2014.05.030

Large wind turbine blades are being developed at lengths of 75–100 m, in order to improve energy capture and reduce the cost of wind energy. Bending loads in the inboard region of the blade make large blade development challenging. The “biplane blade” design was proposed to use a biplane inboard region to improve the design of the inboard region and improve overall performance of large blades. This paper focuses on the design of the internal “biplane spar” structure for 100-m biplane blades. Several spars were designed to approximate the Sandia SNL100-00 blade (“monoplane spar”) and the biplane blade (“biplane spar”). Analytical and computational models are developed to analyze these spars. The analytical model used the method of minimum total potential energy; the computational model used beam finite elements with cross-sectional analysis. Simple load cases were applied to each spar and their deflections, bending moments, axial forces, and stresses were compared. Similar performance trends are identified with both the analytical and computational models. An approximate buckling analysis shows that compressive loads in the inboard biplane region do not exceed buckling loads. A parametric analysis shows biplane spar configurations have 25–35% smaller tip deflections and 75% smaller maximum root bending moments than monoplane spars of the same length and mass per unit span. Root bending moments in the biplane spar are largely relieved by axial forces in the biplane region, which are not significant in the monoplane spar. The benefits for the inboard region could lead to weight reductions in wind turbine blades. Innovations that create lighter blades can make large blades a reality, suggesting that the biplane blade may be an attractive design for large (100-m) blades.


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Tse L.A., Ganapathi G.B., Wirz R.E., Lavine A.S., "Spatial and Temporal Modeling of Sub- and Supercritical Thermal Energy Storage", Solar Energy, Vol. 103, pp. 402–410, May 2014, https://doi.org/10.1016/j.solener.2014.02.040

This paper describes a thermodynamic model that simulates the discharge cycle of a single-tank thermal energy storage (TES) system that can operate from the two-phase (liquid–vapor) to supercritical regimes for storage fluid temperatures typical of concentrating solar power plants. State-of-the-art TES design utilizes a two-tank system with molten nitrate salts; one major problem is the high capital cost of the salts (International Renewable Energy Agency, 2012). The alternate approach explored here opens up the use of low-cost fluids by considering operation at higher pressures associated with the two-phase and supercritical regimes. The main challenge to such a system is its high pressures and temperatures which necessitate a relatively high-cost containment vessel that represents a large fraction of the system capital cost. To mitigate this cost, the proposed design utilizes a single-tank TES system, effectively halving the required wall material. A single-tank approach also significantly reduces the complexity of the system in comparison to the two-tank systems, which require expensive pumps and external heat exchangers. A thermodynamic model is used to evaluate system performance; in particular it predicts the volume of tank wall material needed to encapsulate the storage fluid. The transient temperature of the tank is observed to remain hottest at the storage tank exit, which is beneficial to system operation. It is also shown that there is an optimum storage fluid loading that generates a given turbine energy output while minimizing the required tank wall material. Overall, this study explores opportunities to further improve current solar thermal technologies. The proposed single-tank system shows promise for decreasing the cost of thermal energy storage.


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Mao H-S., Wirz R.E., Goebel D.M., "Plasma Structure of Miniature Ring-Cusp Ion Thruster Discharges", Journal of Propulsion and Power, Vol. 3(3), pp. 628-636, June 2014, https://doi.org/10.2514/1.B34759

Previous miniature ion thruster studies have demonstrated impressive performance using ring-cusp discharges. These studies suggest that the magnetic field must be sufficiently strong to increase primary electron confinement times for ionization, but weak enough to allow plasma electrons to escape and maintain the plasma potential necessary for ionization. To investigate these phenomena, an experiment was developed to allow detailed measurements of the internal structure and characteristics of a miniature ring-cusp discharge. These measurements provide spatially resolved values for plasma density, electron temperature, and plasma potential along a meridian plane. The magnetic field configuration is arranged as a quasi-periodic domain in order to generalize the findings to all multipole discharges. The results show that the magnetic field strength drives the plasma structure, and the dependence on discharge power can be removed with proper scaling of the plasma parameters. The stronger magnetic field results in a higher peak plasma density, but relatively low discharge utilization efficiency. In addition, the potential measurements indicate the likely onset of discharge instability. In contrast, the weaker magnetic field, or baseline configuration, better uses the volume of the chamber. This leads to a higher and more uniform density near the downstream end of the discharge where ion extraction would occur, implying superior discharge utilization.


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Roth-Johnson P., Wirz R.E., "Aero-structural Investigation of Biplane Wind Turbine Blades", Wind Energy, Vol. 17(3), pp. 397-411, March 2014, https://doi.org/10.1002/we.1583

abstract


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Conversano R.W., Wirz R.E., "Mission Capability Assessment of CubeSats Using a Miniature Ion Thruster", Journal of Spacecraft and Rockets, Vol. 50(5), pp. 1035-1046, April 2013, https://doi.org/10.2514/1.A32435

The successful miniaturization of many spacecraft subsystems make CubeSats attractive candidates for evermore-demanding scientific missions. A three-cell CubeSat employing the miniature xenon ion thruster, which features high efficiency and impulse capability, yields a unique spacecraft that can be optimized for a variety of missions ranging from significant inclination changes in a low Earth orbit to lunar transfers. A nominal configuration of a high-Δ𝑉 CubeSat has a dry mass of approximately 6.3 kg, including a 0.75 kg payload, margins, and contingencies. Depending on the thruster and propellant tank configuration, this CubeSat is capable of delivering mission Δ𝑉 values from 1000 to over 7000m/s, enabling low-Earth-orbit inclination change missions and lunar missions. A parametric analysis on a three-cell high-Δ𝑉 CubeSat bus revealed that a range of payload volumes (up to nearly 1.4 units) and masses (up to nearly 6 kg) can be accommodated depending on the Δ𝑉 requirements and mission type. Additionally, this analysis showed that a high-Δ𝑉 three-cell CubeSat in a 600 km low Earth orbit can be designed to provide an inclination change of over 80 deg.


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Araki S.J., Wirz R.E., "Ion–Neutral Collision Modeling Using Classical Scattering with Spin-Orbit Free Interaction Potential", IEEE Transactions on Plasma Science, Vol. 41(3), pp. 470-480, Feb. 2013, https://doi.org/10.1109/TPS.2013.2241457

A particle-in-cell Monte Carlo collision model is developed to explore dominant collisional effects on high-velocity xenon ions incident to a quiescent xenon gas at low neutral pressures. The range of neutral pressure and collisionality examined are applicable for electric propulsion as well as plasma processing devices; therefore, the computational technique described herein can be applied to more complex simulations of those devices. Momentum and resonant charge-exchange collisions between ions and background neutrals are implemented using two different models, classical scattering with spin-orbit free potential and variable-hard-sphere model, depending on the incident particle energy. The primary and charge-exchange ions are tracked separately, and their trajectories within a well-defined “Test Cell” domain are determined. Predicted electrode currents as a function of the Test Cell pressure are compared with electrode currents measured in an ion gun experiment. The simulation results agree well with the experiment up to a Test Cell pressure corresponding to a mean free path of the Test Cell length and then start to deviate with increasing collisionality at higher pressures. This discrepancy at higher pressures is likely due to the increasing influence of secondary electrons emitted from electrodes due to the high-velocity impacts of heavy species (i.e., beam ions and fast neutrals created by charge-exchange interaction) at the electrode surfaces.


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Chu E., Goebel D., Wirz R.E., "Reduction of Energetic Ion Production in Hollow Cathodes by External Gas Injection", Journal of Propulsion and Power, Vol. 29(5), pp. 1155-1163, Oct. 2013, https://doi.org/10.2514/1.B34799

Studies of the hollow-cathode discharge have shown the existence of energetic ions at high-discharge currents that are likely responsible for the high erosion rates observed on the cathode keeper electrode. This work examines the effect of neutral gas injection in the discharge plume of a 250 A lanthanum hexaboride hollow cathode on the production of energetic ions to determine the conditions that yield cathode operation and life. Two different gas injector types are used to deliver neutral gas into the discharge plume and a retarding-potential analyzer is used for ion energy measurements. The flow splits between the cathode internal and external flows, and the number and locations of the external gas injection sites are examined as a function of the discharge current. It is found that increasing discharge current increases the energetic ion production at any given flow rate or injection location. External gas injection reduces energetic ion production for constant cathode flow, with collimated gas-jet injection performing better than distributed injection. Lifetime estimates of the keeper electrode surface due to sputter erosion by ion bombardment reveal that high-discharge current operation at low cathode gas flow produced very energetic ions and limited keeper lifetimes to less than 5000 h. Applying sufficient internal cathode gas flow and external gas injection can extend the keeper life to over 10,000 h at discharge currents of up to 200 A.


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Lakeh R.B., Lavine A.S., Kavehpour H.P., Ganapathi G.B., Wirz R.E., "Effect of Laminar and Turbulent Buoyancy-Driven Flows on Thermal Energy Storage using Supercritical Fluids", Numerical Heat Transfer, Part A, Vol. 64(12), pp. 955–973, Sept. 2013, https://doi.org/10.1080/10407782.2013.811349

Efficient heat transfer to storage fluid is required for the desirable operation of thermal energy storage systems. Most of the fluid candidates for supercritical thermal storage have poor thermal conductivity; therefore, conduction does not provide sufficient heat transfer. The current study concerns a supercritical thermal energy storage system consisting of horizontal tubes filled with a storage fluid in its supercritical state. The results of this study show that the heat transfer to the supercritical fluid is dominated by laminar and turbulent natural convection. The buoyancy-driven flow inside the storage tubes enhances the heat transfer and dramatically reduces the charge time.


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Tarzi Z., Speyer J., Wirz R.E., "Fuel Optimum Low-Thrust Elliptic Transfer using Numerical Averaging", Acta Astronautica, Vol. 86, pp. 95-118, June 2013, https://doi.org/10.1016/j.actaastro.2013.01.003

Low-thrust electric propulsion is increasingly being used for spacecraft missions primarily due to its high propellant efficiency. As a result, a simple and fast method for low-thrust trajectory optimization is of great value for preliminary mission planning. However, few low-thrust trajectory tools are appropriate for preliminary mission design studies. The method presented in this paper provides quick and accurate solutions for a wide range of transfers by using numerical orbital averaging to improve solution convergence and include orbital perturbations. Thus, preliminary trajectories can be obtained for transfers which involve many revolutions about the primary body. This method considers minimum fuel transfers using first-order averaging to obtain the fuel optimum rates of change of the equinoctial orbital elements in terms of each other and the Lagrange multipliers. Constraints on thrust and power, as well as minimum periapsis, are implemented and the equations are averaged numerically using a Gausian quadrature. The use of numerical averaging allows for more complex orbital perturbations to be added in the future without great difficulty. The effects of zonal gravity harmonics, solar radiation pressure, and thrust limitations due to shadowing are included in this study. The solution to a transfer which minimizes the square of the thrust magnitude is used as a preliminary guess for the minimum fuel problem, thus allowing for faster convergence to a wider range of problems. Results from this model are shown to provide a reduction in propellant mass required over previous minimum fuel solutions.


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Mao H-S., Wirz R.E., "Quasi-equilibrium Electron Density Along a Magnetic Field Line", Applied Physics Letters, Vol. 101(22), 224106, Nov. 2012, https://doi.org/10.1063/1.4768301

A methodology is developed to determine the density of high-energy electrons along a magnetic field line for a low- plasma. This method avoids the expense and statistical noise of traditional particle tracking techniques commonly used for high-energy electrons in bombardment plasma generators. By preserving the magnetic mirror and assuming a mixing timescale, typically the elastic collision frequency with neutrals, a quasi-equilibrium electron distribution can be calculated. Following the transient decay, the analysis shows that both the normalized density and the reduction fraction due to collision converge to a single quasi-equilibrium solution.


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Wirz R.E., Anderson J., Katz I., "Time-Dependent Erosion of Ion Optics", Journal of Propulsion and Power, Vol. 27(1), pp. 211-217, Feb. 2011, https://arc.aiaa.org/doi/pdf/10.2514/1.46845

The accurate prediction of ion thruster life requires time-dependent erosion estimates for the ion optics assembly. Such information is critical to end-of-life mechanisms such as electron backstreaming. A two-dimensional ion optics code, CEX2D, was recently modified to handle time-dependent erosion, double ions, and multiple throttle conditions in a single run. The modified code is called CEX2D-T. Comparisons of CEX2D-T results with the NASA solar electric propulsion technology application readiness (NSTAR) thruster life demonstration test and extended life test results show good agreement for both screen and accelerator grid erosion, including important erosion features such as chamfering of the downstream end of the accelerator grid and reduced rate of accelerator grid aperture enlargement with time. The influence of double ions on grid erosion proved to be important for simulating the erosion observed during the NSTAR life demonstration test and extended life test.


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Wirz R.E., Katz I., Goebel D., Anderson J., "Electron Backstreaming Determination for Ion Thrusters", Journal of Propulsion and Power, Vol. 27(1), pp. 206-210, Feb. 2011, https://arc.aiaa.org/doi/pdf/10.2514/1.46844

Electron backstreaming in ion thrusters is caused by the random flux of beam electrons past a potential barrier established by the accelerator grid. A technique that integrates this flux over the radial extent of the barrier reveals important aspects of electron backstreaming phenomena for individual beamlets, across the thruster beam, and throughout thruster life. For individual beamlets it was found that over 99% of the electron backstreaming occurs in a small area at the center of the beamlet that is less than 20% the area of the beamlet at the potential barrier established by the accelerator grid. For the thruster beam it was found that over 99% of the backstreaming current occurs inside of r = 6 cm for the over 28 cm diameter NSTAR grid. Initial validation against extended life test data for the NSTAR thruster shows that the technique provides the correct behavior and magnitude of electron backstreaming limit, 𝑉 𝑏 𝑠 V bs ​ . From the sensitivity analyses it is apparent that accelerator grid chamfering due to sputter erosion contributes significantly to the sharp rise in electron backstreaming limit observed in the extended life test, but does not explain the rise in grid ion transparency. Reduction of the grid gap over the life of the thruster also contributes to increases in electron backstreaming limit and increases in ion transparency. Screen grid erosion contributes generally to rises in 𝑉 𝑏 𝑠 V bs ​ and grid ion transparency, but for the assumptions used herein, it appears to not have as much of an effect as chamfering or grid gap change. Overall, it is apparent that accelerator grid chamfering, grid gap change, and screen grid erosion are important to the increase in electron backstreaming observed during the NSTAR extended life test.


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Wirz R.E., Shariff S., "Ground Effects for Widely Spaced, Supersonic Vertical Retrorockets on CEV", Journal of Spacecraft and Rockets, Vol. 46(3), pp. 599-605, June 2009, https://doi.org/10.2514/1.36299

A scenario for creating acceptable touchdown velocity on land for the Orion Crew Excursion Vehicle employs retrorockets for final landing A.V. To capture the ground effects due to the interaction of the retrorockets and the vehicle, detailed computational modeling was used to determine the effective thrust at several different firing heights. These results were then used to determine the change in impulse for a wide range of possible firing altitudes. For a Crew Excursion Vehicle retrorocket firing time of 0.5 s, the steady-state effective vertical thrust of the module changes from –13.2 to +11.8% for altitudes from 152 to 15.2 cm, respectively. A simple descent analysis shows that ground effects will impart a net decrease or increase in impulse, depending on ignition altitude. In this analysis, the ground effects serve to increase the optimal firing height and increase the ignition altitude margin for a given maximum landing velocity.


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Wirz R.E., Gamero M., Goebel D., "Pulsed Operation of an Ion Accelerator", NASA Tech Brief, Vol. 33(2), NPO-44961, Feb. 2009, https://ntrs.nasa.gov/search.jsp?R=20090008642

Electronic circuitry has been devised to enable operation of an ion accelerator in either a continuous mode or a highpeak power, low-average-power pulsed mode. In the original intended application, the ion accelerator would be used as a spacecraft thruster and the pulse mode would serve to generate small increments of impulse for precise control of trajectories and attitude. The present electronic drive circuitry generates the extraction voltage in pulses. Pulse-width modulation can affect rapid, fine control of time-averaged impulse or ion flux down to a minimum level much lower than that achievable in continuous operation.


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Hofer R.R., Johnson L.K., Goebel D.M., Wirz R.E., "Effects of Internally Mounted Cathodes on Hall Thruster Plume Properties", IEEE Transactions on Plasma Science, Vol. 36(5), pp. 2004-2014, Oct. 2008, https://doi.org/10.1109/TPS.2008.2000962

The effects of cathode position on the operation and plume properties of an 8-kW Hall thruster are discussed. Thruster operation was investigated at operating conditions ranging from 200 to 500 V of discharge voltage, 10-40 A of discharge current, and 2-8 kW of discharge power, with a cathode positioned either in the traditional externally mounted configuration outside the outer magnetic pole piece or in an internally mounted configuration central to the inner magnetic core. With the external cathode, substantial emission in the visible spectrum that follows magnetic field lines surrounds the exterior pole pieces of the thruster. With the internal cathode, the emission is largely absent while the cathode plume is compressed and elongated in the axial direction by the strong axial magnetic field on the thruster centerline. Discharge current oscillation and ion species fraction measurements were found to be similar for the cathode locations, whereas the operation with the internal cathode was found to favor an improved coupling of the cathode plume with the thruster discharge. Ion current density measurements show that with respect to externally mounted designs, internally mounted cathodes reduce plume divergence and increase the symmetry of the near-field plume. The impacts of internally mounted cathodes on thruster physics and spacecraft integration activities are assessed.


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Wirz R.E., Anderson J.R., Goebel D.M., Katz I., "Decel Grid Effects on Ion Thruster Grid Erosion", IEEE Transactions on Plasma Science, Vol. 36(5), pp. 2122-2129, Oct. 2008, https://doi.org/10.1109/TPS.2008.2001041

Jet Propulsion Laboratory (JPL) is currently assessing the applicability of the 25-cm Xenon Ion Propulsion System (XIPS) as part of an effort to infuse low-cost technically mature commercial ion thruster systems into NASA deep space missions. Since these mission require extremely long thruster lifetimes to attain the required mission DeltaV, this paper is focused on understanding the dominate wear mechanisms that effect the life of the XIPS three-grid system. Analysis of the XIPS three-grid configuration with JPL's CEX3D grid erosion model shows that the third ldquodecelrdquo grid effectively protects the accel grid from pits and grooves erosion that is commonly seen with two-grid ion thruster grid systems. For a three-grid system, many of the charge-exchange ions created downstream of the grid plane will impact the decel grid at relatively low energies ( ~25 V), instead of impacting the accel grid at high energies ( ~200 V), thus reducing overall erosion. JPL's CEX3D accurately predicts the erosion patterns for the accel grid, although it appears to overpredict the pits and grooves erosion rates due, mainly, to uncertainties in incident energies and angles for sputtering ions and since it does not account for local redeposition of sputtered material. Since the model accurately simulates the erosion pattern but tends to overpredict the erosion rates for both the two- and three-grid sets, this comparative analysis clearly shows how the decel grid significantly suppresses the erosion of the downstream surface of the accel grid as observed in experimental tests. The results also show that the decel grid has a relatively small effect on barrel erosion (erosion of the aperture wall) and erosion of the upstream surface of the accel grid. Decreasing the accel grid voltage of the XIPS can reduce barrel (and total) erosion of the accel grid and should be considered for high-DeltaV missions.


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Wirz R.E., Goebel D.M., "Effects of Magnetic Field Topography on Ion Thruster Discharge Performance", Plasma Sources Science and Technology, Vol. 17, 035010, June 2008, https://doi.org/10.1088/0963-0252/17/3/035010

Traditional magnetic field design techniques for dc ion thrusters typically focus on closing a sufficiently high maximum closed magnetic contour, Bcc, inside the discharge chamber. In this study, detailed computational analysis of several modified NSTAR thruster 3-ring and 4-ring magnetic field geometries reveals that the magnetic field line shape as well as Bcc determines important aspects of dc ion thruster performance (i.e. propellant efficiency, beam flatness and double ion content). The DC-ION ion thruster model results show that the baseline NSTAR configuration traps the primary electrons on-axis, which leads to the high on-axis plasma density peak and high double ion content observed in experimental measurements. These problems are further exacerbated by simply increasing Bcc and not changing the field line shape. Changing the field line shape to prevent on-axis confinement (while maintaining the NSTAR baseline Bcc) improves thruster performance, improves plasma uniformity and lowers double ion content. For these favorable field line geometries, we observe further improvements to performance with increased Bcc, while maintaining plasma uniformity and low double ion content. These improvements derive from the fact that the field lines guide the high-energy primaries to regions where they are most efficiently used to create ions while a higher Bcc prevents the loss of ions to the anode walls. Therefore, it is recommended that the ion thruster designer first establish a divergent field line shape that ensures favorable beam flatness, low double ion content and reasonable performance; then the designer may adjust the Bcc to attain desirable performance and stability for the target discharge plasma conditions.


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Wirz R.E., Sullivan R., Przybylowski J., Silva M., "Hollow Cathode and Low-Thrust Extraction Grid Analysis for a Miniature Ion Thruster", International Journal of Plasma Science and Engineering, 2008, pp. 1-11, June 2008, http://dx.doi.org/10.1155/2008/693825

Miniature ion thrusters are well suited for future space missions that require high efficiency, precision thrust, and low contamination in the mN to sub-mN range. JPL's miniature xenon Ion (MiXI) thruster has demonstrated an efficient discharge and ion extraction grid assembly using filament cathodes and the internal conduction (IC) cathode. JPL is currently preparing to incorporate a miniature hollow cathode for the MiXI discharge. Computational analyses anticipate that an axially upstream hollow cathode location provides the most favorable performance and beam profile; however, the hot surfaces of the hollow cathode must be sufficiently downstream to avoid demagnetization of the cathode magnet at the back of the chamber, which can significantly reduce discharge performance. MiXI's ion extraction grids are designed to provide >3 mN of thrust; however, previous to this effort, the low-thrust characteristics had not been investigated. Experimental results obtained with the MiXI-II thruster (a near replica or the original MiXI thruster) show that sparse average discharge plasma densities of ∼5×1015–5×1016 m-3 allow the use of very low beamlet focusing extraction voltages of only ∼250–500 V, thus providing thrust levels as low as 0.03 mN for focused beamlet conditions. Consequently, the thrust range thus far demonstrated by MiXI in this and other tests is 0.03–1.54 mN.


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Goebel D.M., Polk J.E., Sengupta A., Wirz R.E., "Increasing the Life of a Xenon-Ion Spacecraft Thruster", NASA Tech Brief, Vol. 31(11), NPO-43495, Nov. 2007, https://ntrs.nasa.gov/search.jsp?R=20100011234

A short document summarizes the redesign of a xenon-ion spacecraft thruster to increase its operational lifetime beyond a limit heretofore imposed by nonuniform ion-impact erosion of an accelerator electrode grid. A peak in the ion current density on the centerline of the thruster causes increased erosion in the center of the grid. The ion-current density in the NSTAR thruster that was the subject of this investigation was characterized by peak-to-average ratio of 2:1 and a peak-to-edge ratio of greater than 10:1. The redesign was directed toward distributing the same beam current more evenly over the entire grid andinvolved several modifications of the magnetic- field topography in the thruster to obtain more nearly uniform ionization. The net result of the redesign was to reduce the peak ion current density by nearly a factor of two, thereby halving the peak erosion rate and doubling the life of the thruster.


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Goebel D., Wirz R.E., Katz I., "Analytical Ion Thruster Discharge Performance Model", Journal of Propulsion and Power, Vol. 23(5), pp. 1055-1067, Oct. 2007, https://doi.org/10.2514/1.26404

A particle and energy balance model of the plasma discharge in magnetic ring-cusp ion thrusters has been developed. The model follows prior work in the development of global zero-dimensional discharge models that use conservation of particles into and out of the thruster, conservation of energy into the discharge and out of the plasma in the form of charged particles to the walls and beam, and plasma radiation. The present model is significantly expanded over the prior art by closing the set of equations with self-consistent calculations of the internal neutral pressure, electron temperature, primary electron density, electrostatic ion confinement (due to the ring-cusp fields), plasma potential, discharge stability, and time-dependent behavior during recycling. The model only requires information on the thruster geometry, ion optics performance, and electrical inputs, such as discharge voltage and currents, to produce accurate performances curves of discharge loss vs mass utilization efficiency. The model has been benchmarked against several ion thrusters, and successfully predicts the thruster discharge loss as a function of mass utilization efficiency for a variety of thrusters. The discharge performance model will be described and results showing ion thruster performance and stability presented.


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221C Dearborn Hall

Corvallis, OR 97331